Fix to validate nunjucksPaths as a string

lib/plugins/plugin-validator.js

@@ -40,9 +40,13 @@ function checkNunjucksMacroExists (executionPath, nunjucksFileName, nunjucksPath
   // set up a flag for the existance of a nunjucks path
   let nunjucksPathExists = false
 
-  if (nunjucksPaths === undefined) {
-    // Check if the nunjucksMacros are at the root level of the project
-    if (fse.existsSync(path.join(executionPath, nunjucksFileName))) nunjucksPathExists = true
+  if (!nunjucksPaths || typeof nunjucksPaths === 'string') {
+    const pathToCheck = typeof nunjucksPaths === 'string'
+      ? path.join(executionPath, nunjucksPaths, nunjucksFileName)
+      : path.join(executionPath, nunjucksFileName) // root level
+
+    // Check if the nunjucksMacros are at a single path in the project
+    if (fse.existsSync(pathToCheck)) nunjucksPathExists = true
   } else {
     // Combine the file path name for each nunjucks path and check if any one of them exists
     for (const nunjucksPath of nunjucksPaths) {