Add null check in MatchLegacySwitchOnStringWithDict

[ds5678]

Updated the condition for nullValueCaseBlock to ensure it is not null and not equal to defaultBlock.

Problem

Resolves #3421

Solution

• Any comments on the approach taken, its consistency with surrounding code, etc.
• Which part of this PR is most in need of attention/improvement?
• At least one test covering the code changed
  • Is a test necessary for this? If so, I'm not sure what the best way to write that test is.

[siegfriedpammer]

Thank you very much for looking into this problem and providing a fix!

I think a test case would be nice, but is not strictly necessary. Judging from the .NET Framework version, this is code generated by the C# 2.0 compiler and I think you could only add an IL pretty test, similar to the test in your last PR.

The fix is very simple and fixing an obvious flaw, if you look at the code that handles C# 1.0 switch on string (using Hashtable), you can see that a check for Leave instructions (where the defaultBlock would be null) is present there, but not in the C# 2.0 case.