Empty query in extended query

[sunng87]

Currently we don't deal with empty query in extended query like what we do for simple query protocol. This will result in error on query parse and execution.

[sunng87]

This is standard behavior of empty query in extended protocol

Sent Parse message - Query: '' (empty)
Sent Describe Statement message
Sent Bind message
Sent Describe Portal message
Sent Execute message
Sent Sync message
Received ParseComplete
Received ParameterDescription: 0 parameters
Received NoData (no parameters or result columns)
Received BindComplete
Received NoData (no parameters or result columns)
Received EmptyQueryResponse - Empty query executed successfully!
Received ReadyForQuery. Transaction status: I

The responses for parse/describe statement/bind/describe portal/execute:

PostgreSQL
    Type: Parse completion
    Length: 4
PostgreSQL
    Type: Parameter description
    Length: 6
    Parameters: 0
PostgreSQL
    Type: No data
    Length: 4
PostgreSQL
    Type: Bind completion
    Length: 4
PostgreSQL
    Type: No data
    Length: 4
PostgreSQL
    Type: Empty query
    Length: 4
PostgreSQL
    Type: Ready for query
    Length: 5
    Status: Idle (73)

[sunng87]

Actually we already have support for these in DescribeResponse. We may need to support it at API level by changing Statement from QueryParser to Option<Statement>.