Fix for #365

Parsing a [null] parameter in for example logger.Info("Testing null:
{0}", null); will result in not logging an event. A
System.NullReferenceException would be thrown causing the entire
logEvent  NOT to be logged. This behavior is different from prior
versions. This fix will restore the previous behavior.
Added test

Added unit test  for fix #365
Altered null check
Merge pull request #366 from cvanbergen/issue365

Fix for #365: Behaviour when logging null arguments