fix(quality): improve theme guessing reliability

See report by SonarQube https://sonarcloud.io/project/issues?severities=BLOCKER%2CCRITICAL%2CMAJOR%2CMINOR&sinceLeakPeriod=true&issueStatuses=OPEN%2CCONFIRMED&types=BUG&pullRequest=437&id=Guts_mkdocs-rss-plugin&open=AZ2Xkk5h3GBU49WbRb1I

Author: Guts

mkdocs_rss_plugin/integrations/theme_material_base.py

@@ -33,7 +33,7 @@
 class IntegrationMaterialThemeBase:
     # attributes
     IS_THEME_MATERIAL: bool = False
-    THEME_NAME: str = ""
+    THEME_NAME: str = "mkdocs"
 
     def __init__(self, mkdocs_config: MkDocsConfig) -> None:
         """Integration instantiation.
@@ -60,7 +60,18 @@ def is_mkdocs_theme_material(
         if mkdocs_config is None and isinstance(self.mkdocs_config, MkDocsConfig):
             mkdocs_config: MkDocsConfig = self.mkdocs_config
 
-        self.THEME_NAME = mkdocs_config.theme.name if mkdocs_config.theme else ""
-
-        self.IS_THEME_MATERIAL = mkdocs_config.theme.name in ("material", "materialx")
-        return self.IS_THEME_MATERIAL
+        if isinstance(mkdocs_config, MkDocsConfig):
+            self.THEME_NAME = (
+                mkdocs_config.theme.name if mkdocs_config.theme else "mkdocs"
+            )
+            self.IS_THEME_MATERIAL = mkdocs_config.theme.name in (
+                "material",
+                "materialx",
+            )
+            return self.IS_THEME_MATERIAL
+
+        logger.warning(
+            "Cannot check if the theme is Material or not because the MkDocs "
+            "configuration object is not available."
+        )
+        return False