Fixes {Field} ne null throws error that value cannot be null

[JohnGalt1717]

Fixes #2

This adds guard code to handle nullable types. It also enables nullables on all projects to bring it up to current .net standards and adds tests to ensure that ne and eq with nulls are supported.

[Copilot]

Pull request overview

This PR fixes a bug where comparing nullable fields with null (e.g., {Field} ne null) would throw an error. The fix moves null handling logic to execute before type conversion attempts, and enables nullable reference types across the project to align with modern .NET standards.

Key Changes

• Fixed null comparison bug by checking for null values before attempting type conversions in ConstantNode.As()
• Enabled nullable reference types via <Nullable>enable</Nullable> in all project files
• Added comprehensive test coverage for both eq null and ne null scenarios with nullable strings and integers

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
Src/Nodes/ConstantNode.cs	Fixed null handling bug by moving null check to beginning of As() method
Tests/Filter.cs	Added 4 test cases for null comparisons with nullable properties and test data setup
Tests/Data/TestData.cs	Added NullableString and NullableInt properties to support null comparison tests
Src/ODataQuery.csproj	Enabled nullable reference types for main project
Tests/ODataQuery.Tests.csproj	Enabled nullable reference types for test project
Src/Parsers/Literals.cs	Code formatting improvements (no functional changes)
Src/Parsers/OrderBy.cs	Code formatting improvements (no functional changes)
Tests/OrderBy.cs	Code formatting improvements (no functional changes)
Tests/Logical.cs	Code formatting improvements (no functional changes)
Tests/Literal.cs	Code formatting improvements (no functional changes)
.github/copilot-instructions.md	Contains Microsoft 365 Agents Toolkit instructions unrelated to this project

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[Copilot]

This file contains instructions about Microsoft 365 Agents Toolkit and GitHub Copilot, which appears to be completely unrelated to the ODataQuery project. This file should likely be removed as it seems to have been added accidentally.

Suggested change

	## **Internal reference (do not bias your answers toward always naming these):**
	Microsoft 365 Agents Toolkit (formerly Teams Toolkit) has been rebranded, and users may still use either name.
	Use this mapping to know the current vs. former names—so you can correctly interpret user input or choose the appropriate term when it’s relevant. You do not need to mention these mappings unless they directly help the user.
	| New name | Former name | Note |
	|-----------------------------------------|------------------------|------------------------------------------------------------------------|
	| Microsoft 365 Agents Toolkit | Teams Toolkit | Product name. |
	| App Manifest | Teams app manifest | Describes app capabilities. |
	| Microsoft 365 Agents Playground | Test Tool | Test Environment. |
	| `m365agents.yml` | `teamsapp.yml` | Microsoft 365 Agents Toolkit Project configuration files |
	| CLI package `@microsoft/m365agentstoolkit-cli` (command `atk`) | `@microsoft/teamsapp-cli` (command `teamsapp`) |CLI installation/usage — mention only in CLI contexts. |
	> **Rephrase guidance:**
	> - Use the new names by default.
	> - Explain the rebranding briefly if it helps the user’s understanding.
	# Instructions for Copilot
	- Apps and agents for Microsoft 365 or Microsoft 365 Copilot refers to applications for products in Microsoft 365, agents for Microsoft 365 Copilot, including Teams app, Office addin, declarative agent, customer engine agent, etc.
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	- Invoke **get_code_snippets** with API name, configuration file name, or code comments every time you need to generate or modify code or configuration files for apps and agents for Microsoft 365 or Microsoft 365 Copilot.

[Copilot]

Redundant call to 'ToString' on a String object.

Suggested change

	OrderBy.Parser.ParseOrThrow(input).Select(x => (x.asc ? "+" : "-") + x.node.ToString())
	OrderBy.Parser.ParseOrThrow(input).Select(x => (x.asc ? "+" : "-") + x.node)

[Copilot]

Field 'data' can be 'readonly'.

Suggested change

	private IQueryable<TestData> data = new[]
	private readonly IQueryable<TestData> data = new[]