Clang: Improve ProjectPartArtefact

Empty strings were only handled by accident and wrongly formatted ones
were never handled. Now we do nothing for empty strings and throw an
exception from wrongly formatted strings. The exceptions are very helpful
in the test code because the show errors the the testing data.

Change-Id: I87d1678eda7502fdc3f74f51fad491803d28582b
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>

tests/unit/unittest/symbolstorage-test.cpp

@@ -90,7 +90,7 @@ protected:
                                 {2, {"function2USR", "function2"}}};
     SourceLocationEntries sourceLocations{{1, {1, 3}, {42, 23}, SymbolType::Declaration},
                                           {2, {1, 4}, {7, 11}, SymbolType::Declaration}};
-    ClangBackEnd::ProjectPartArtefact artefact{"-DFOO", "{\"FOO\":\"1\"}", 74};
+    ClangBackEnd::ProjectPartArtefact artefact{"[\"-DFOO\"]", "{\"FOO\":\"1\"}", 74};
     Storage storage{statementFactory, filePathCache};
 };