[lldb] Fix a bug in the decorator matching logic.

This changes the decorator helper `_match_decorator_property` to
consider `None` as the actual value as not a match. Using `None` for the
pattern continues to be considered a match.

I discovered the issue because marking a test as NO_DEBUG_INFO_TESTCASE
will cause the call to `self.getDebugInfo()` to return `None` and
incorrectly skip or XFAIL the corresponding test.

I used the above scenario to create a test for the decorators.

Differential revision: https://reviews.llvm.org/D123401