mtest: early return when parsing blank lines in TAP

Just like comment lines, blank lines do nothing. Before commit
a7e458effa we ended off the parser by
returning if the line was blank, because we needed to in order to catch
non-blank lines as errors. But really, we should have always returned
much earlier and not wasted time attempting to process anything.

mesonbuild/mtest.py

@@ -380,7 +380,7 @@ class TAPParser:
                 self.state = self._MAIN
 
             assert self.state == self._MAIN
-            if line.startswith('#'):
+            if not line or line.startswith('#'):
                 return
 
             m = self._RE_TEST.match(line)