Ensure that 0 length reads return an empty array not null

[Wraith2]

fixes #3871

There is an edge case int TdsParserStateObject.TryReadPlpBytes in continue enabled mode where the attempt to fetch the snapshot stored buffer when there is none available causes the return of a null array reference and not an empty array. This introduces a temp variable and only uses the stored buffer in this case if it is non-null.

@dotnet/sqlclientdevteam can you run the CI please

/cc @kaiohenrique

[mdaigle]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[kaiohenrique]

Hello, @Wraith2
Should we also apply the fix here https://github.com/Wraith2/SqlClient/blob/fix-3871/src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/TdsParser.cs#L13780?

[kaiohenrique]

In the storage, is it possible the buffer is incompatible with the column datatype being read at the moment? I am thinking in something like that

Previous column stored byte[] array in snapshot (from VARCHAR data)
Empty NVARCHAR tries to retrieve char[] from snapshot
Cast fails => returns NULL

OR

Previous column stored char[] array in snapshot (from NVARCHAR data)
Empty VARCHAR tries to retrieve byte[] from snapshot
Cast fails => returns NULL

[Wraith2]

Hello, @Wraith2 Should we also apply the fix here https://github.com/Wraith2/SqlClient/blob/fix-3871/src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/TdsParser.cs#L13780?

If you can produce an example of it causing a failure. This code is both performance sensitive and very complex to reason about, i'd rather not make changes without good reason.