Add comprehensive tests for parseConfiguration function, handle non-object parser results consistently (fixes #1523).

lib/markdownlint.mjs

@@ -1183,6 +1183,7 @@ export function readConfigSync(file, parsers, fs) {
   // Try to parse file
   const { config, message } = parseConfiguration(file, content, parsers);
   if (!config) {
+    // @ts-ignore
     throw new Error(message);
   }
   // Extend configuration

lib/parse-configuration.mjs

@@ -5,7 +5,7 @@
  *
  * @typedef {Object} ParseConfigurationResult
  * @property {Object | null} config Configuration object if successful.
- * @property {string} message Error message if an error occurred.
+ * @property {string | null} message Error message if an error occurred.
  */
 
 /**
@@ -18,20 +18,24 @@
  */
 export default function parseConfiguration(name, content, parsers) {
   let config = null;
-  let message = "";
+  let message = null;
   const errors = [];
   let index = 0;
   // Try each parser
-  (parsers || [ JSON.parse ]).every((parser) => {
+  const failed = (parsers || [ JSON.parse ]).every((parser) => {
     try {
-      config = parser(content);
+      const result = parser(content);
+      config = (result && (typeof result === "object") && !Array.isArray(result)) ? result : {};
+      // Succeeded
+      return false;
     } catch (error) {
       errors.push(`Parser ${index++}: ${error.message}`);
     }
-    return !config;
+    // Failed, try the next parser
+    return true;
   });
   // Message if unable to parse
-  if (!config) {
+  if (failed) {
     errors.unshift(`Unable to parse '${name}'`);
     message = errors.join("; ");
   }