gdbus-codegen: If writing body to stdout, don't try to include header

If we're writing the body to standard output, we cannot know what the
filename of the corresponding header is going to be, but it seems
vanishingly unlikely that it will be either `stdout.h` (which we would
traditionally have generated) or `-.h` (which we would have generated
since !3886).

This makes some of the output snippets sufficiently short that black(1)
requires that they are folded into a single line.

Signed-off-by: Simon McVittie <smcv@collabora.com>

gio/gdbus-2.0/codegen/codegen_main.py

@@ -336,7 +336,11 @@ def codegen_main():
             print_error("Using --body requires --output")
 
         c_file = args.output
-        header_name = os.path.splitext(os.path.basename(c_file))[0] + ".h"
+
+        if c_file == "-":
+            header_name = ""
+        else:
+            header_name = os.path.splitext(os.path.basename(c_file))[0] + ".h"
     elif args.interface_info_header:
         if args.output is None:
             print_error("Using --interface-info-header requires --output")
@@ -358,7 +362,11 @@ def codegen_main():
             )
 
         c_file = args.output
-        header_name = os.path.splitext(os.path.basename(c_file))[0] + ".h"
+
+        if c_file == "-":
+            header_name = ""
+        else:
+            header_name = os.path.splitext(os.path.basename(c_file))[0] + ".h"
 
     # Check the minimum GLib version. The minimum --glib-min-required is 2.30,
     # because that’s when gdbus-codegen was introduced. Support 1, 2 or 3