Added some tests for PropertySearchResults

New tests:
+ `shouldReturnEmptySearchResultsMessage`: Creates a `PropertySearchResultsBuilder` with a null error message, then builds the search results as `PropertySearchResults`, checks for the expected error message.
+ `shouldReturnNullSearchResultsList`:  Creates a `PropertySearchResultsBuilder`, then builds the search results as `PropertySearchResults`, checks to make sure the return value of `getPropertyList()` is `null`.