Assign the option if the value is defined

Author: ggrossetie

lib/options.js

@@ -72,8 +72,12 @@ const convertOptions = (args, attrs) => {
   if (destinationDir != null) {
     options.to_dir = destinationDir
   }
-  options.template_dirs = templateDir
-  options.template_engine = templateEngine
+  if (templateDir) {
+    options.template_dirs = templateDir
+  }
+  if (templateEngine) {
+    options.template_engine = templateEngine
+  }
   if (typeof outFile !== 'undefined') {
     if (outFile === '') {
       options.to_file = '-'