Fix NullReferenceException in DbConnectionOptions.ConvertValueToBoolean

[pumpkin-bit]

Problem/bug

In the System.Data.Common component, the DbConnectionOptions class is responsible for parsing connection strings. If a connection string specifies a key without a value, the parser stores that key in the internal _parsetable dictionary with a null value. Inside the ConvertValueToBooleanInternal method, stringValue.Trim() was called; since stringValue was null, the program crashed with an unhandled NullReferenceException instead of throwing the appropriate exception.

The code originally contained a comment: // TODO: Is it possible for _parsetable to contain a null value here? If so there's a bug here, investigate..

---

Solution

A check was added to ConvertValueToBooleanInternal if stringValue is null, the standard ADP.InvalidConnectionOptionValue(keyName) (ArgumentException) is used immediately. The method signature was changed to accept string? stringValue to explicitly support nullable strings, and an obsolete TODO comment was removed.
A unit test was added to DbConnectionStringBuilderTest.cs to verify handling of empty key values. All tests passed.

[azure-pipelines]

Azure Pipelines:
Successfully started running 3 pipeline(s).
13 pipeline(s) were filtered out due to trigger conditions.
There may be pipelines that require an authorized user to comment /azp run to run.

[dotnet-policy-service]

Tagging subscribers to this area: @SamMonoRT, @dotnet/efteam
See info in area-owners.md if you want to be subscribed.