meldapp: Don't try to open a blank comparison tab (#193)

We have existing code in meldapp that handles this 90% fine. However,
if the user does something like always using `--diff` (which is
particularly understandable for e.g., tooling) then invocations like:
    meld -n --diff a b
will result in the new tab not being focused on open, because it's not
first in the list of comparisons we're opening.

meld/meldapp.py

@@ -333,7 +333,7 @@ class MeldApp(Gtk.Application):
 
         tab = None
         error = None
-        comparisons = [args] + options.diff
+        comparisons = [c for c in [args] + options.diff if c]
 
         # Every Meld invocation creates at most one window. If there is
         # no existing application, a window is created in do_startup().