docs: only add configuration files to sitemap once generated

The generation script can fail to generate a configuration for the four
libraries where the documentation is not generated from gir, as it needs
to run pkg-config.

Only add the path to the configuration file to the sitemap once it has
successfully been dumped.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8324>

subprojects/gstreamer/docs/gst-plugins-doc-cache-generator.py

@@ -155,7 +155,6 @@ class GstLibsHotdocConfGen:
             project_name = PROJECT_NAME_MAP.get(libname, libname)
 
             conf_path = self.builddir / f'{project_name}-doc.json'
-            conf_files.append(str(conf_path))
 
             index_path = os.path.join(self.source_root, 'index.md')
             if not os.path.exists(index_path):
@@ -171,7 +170,6 @@ class GstLibsHotdocConfGen:
             if not os.path.exists(c_index_path):
                 c_index_path = index_path
 
-
             try:
                 if libname == 'adaptivedemux':
                     c_flags = get_c_flags(f'gstreamer-base-{self.project_version}', self.buildroot)
@@ -216,6 +214,8 @@ class GstLibsHotdocConfGen:
             with conf_path.open('w') as f:
                 json.dump(conf, f, indent=4)
 
+            conf_files.append(str(conf_path))
+
 
         if self.output is not None:
             with self.output.open('w') as f: