Actually default the XTerm title if the version is there but empty

Author: davep

src/textual_dev/tools/diagnose.py

@@ -87,7 +87,7 @@ def _guess_term() -> str:
         elif "GNOME_TERMINAL_SCREEN" in os.environ:
             term_program = "GNOME Terminal"
         elif "XTERM_VERSION" in os.environ:
-            term_program = os.environ.get("XTERM_VERSION", "XTerm")
+            term_program = os.environ.get("XTERM_VERSION") or "XTerm"
 
     else:
         # See if we can pull out some sort of version information too.