TargetPlatformMoniker should be blank if TargetPlatformIdentifier is not set

[dsplaisted]

#5391 made it possible for the TargetPlatformIdentifier not to be set.

However, the TargetPlatformMoniker is still set in Microsoft.Common.CurrentVersion.targets if it's not already set:

msbuild/src/Tasks/Microsoft.Common.CurrentVersion.targets

Line 99 in e8c17c1

<TargetPlatformMoniker Condition="'$(TargetPlatformMoniker)' == ''">$(TargetPlatformIdentifier),Version=$(TargetPlatformVersion)</TargetPlatformMoniker>

So when the TargetPlatformIdentifier is blank, the TargetPlatformMoniker is set to ,Version=.

We should update the logic so that if the TargetPlatformIdentifier is blank, the TargetPlatformMoniker won't be set.

[dsplaisted]

@sfoslund