Fix NRE in DiagnosticSourceListener when property values are null

[pharring]

The DiagnosticSourceListener tests project got removed with the last round of project shuffling (#135)
This puts it back.

Also fix a bug (NRE) when property values are null and add a test to cover that case.

[SergeyKanzhelev]

@pharring build failed with the error. Can you please fix?

3 error(s), 10 warning(s)

Build - 3 error(s), 10 warning(s)
src\DiagnosticSourceListener\DiagnosticSourceListenerSubscription.cs(61):src\DiagnosticSourceListener\DiagnosticSourceListenerSubscription.cs(61,0): Error CA1305: Microsoft.Globalization : Because the behavior of 'Convert.ToString(object)' could vary based on the current user's locale settings, replace this call in 'DiagnosticSourceListenerSubscription.OnNext(KeyValuePair<string, object>)' with a call to 'Convert.ToString(object, IFormatProvider)'. If the result of 'Convert.ToString(object, IFormatProvider)' will be displayed to the user, specify System.Globalization.CultureInfo.CurrentCulture as the 'IFormatProvider' parameter. Otherwise, if the result will be stored and accessed by software, such as when it is persisted to disk or to a database, specify System.Globalization.CultureInfo.InvariantCulture.
src\DiagnosticSourceListener\DiagnosticSourceListenerSubscription.cs(61):src\DiagnosticSourceListener\DiagnosticSourceListenerSubscription.cs(61,0): Error CA1305: Microsoft.Globalization : Because the behavior of 'Convert.ToString(object)' could vary based on the current user's locale settings, replace this call in 'DiagnosticSourceListenerSubscription.OnNext(KeyValuePair<string, object>)' with a call to 'Convert.ToString(object, IFormatProvider)'. If the result of 'Convert.ToString(object, IFormatProvider)' will be displayed to the user, specify 'CultureInfo.CurrentCulture' as the 'IFormatProvider' parameter. Otherwise, if the result will be stored and accessed by software, such as when it is persisted to disk or to a database, specify 'CultureInfo.InvariantCulture'.
Process 'msbuild.exe' exited with code '1'.

[pharring]

@SergeyKanzhelev ugh! Sorry about that - I should have done a Release build.