Removed bogus null checks

* Removed null checks in config extensions if sink factory returns null before calling Sink() method from Serilog's LoggerSinkConfiguration or LoggerAuditSinkConfiguration. The truth is that our sink factory may not return null. If it does in production, we should let the exception happen. It happened in the tests because of a missing setup on two mocks.
* Added missing setups in tests.

Author: ckadluba

src/Serilog.Sinks.MSSqlServer/Configuration/Extensions/Hybrid/LoggerConfigurationMSSqlServerExtensions.cs

@@ -164,8 +164,6 @@ internal static LoggerConfiguration MSSqlServerInternal(
 
             var periodicBatchingSink = batchingSinkFactory.Create(sink, sinkOptions);
 
-            if (periodicBatchingSink == null) return null;
-
             return loggerConfiguration.Sink(periodicBatchingSink, restrictedToMinimumLevel, sinkOptions?.LevelSwitch);
         }
 
@@ -283,8 +281,6 @@ internal static LoggerConfiguration MSSqlServerInternal(
 
             var auditSink = auditSinkFactory.Create(connectionString, sinkOptions, formatProvider, columnOptions, logEventFormatter);
 
-            if (auditSink == null) return null;
-
             return loggerAuditSinkConfiguration.Sink(auditSink, restrictedToMinimumLevel, sinkOptions?.LevelSwitch);
         }