lupdate records qtTrId() calls even if no string template is specified

Running lupdate on a main.cpp that simply contains

qtTrId("foo");

will produce a translation file with an entry for "foo", with an empty source tag.
lupdate should probably warn when no string template has been specified, and not create the translation entry.

A separate, but related, issue is what to do if the string template is missing but some other meta-data has been specified.