Fix srcToken error

Author: mdbraber

src/rules/yaml-key-sort.ts

@@ -63,6 +63,10 @@ export default class YamlKeySort extends RuleBuilder<YamlKeySortOptions> {
     const doc = parseYAML(yamlText);
     const startingPriorityKeys = getEmptyDocument(doc);
 
+	if(doc.contents == null) {
+		return text;
+	}
+
     let remainingKeys = this.getYAMLKeysSorted(yamlKeys, doc, startingPriorityKeys);
 
     const sortOrder = options.yamlSortOrderForOtherKeys;