fix: typescript declaration of titleTemplate (#603)

fix #527

Author: nstringham

types/vue-meta.d.ts

@@ -172,7 +172,7 @@ export interface NoScriptProperty extends MetaDataProperty {
 
 export interface MetaInfo {
   title?: string
-  titleTemplate?: string | ((titleChunk: string) => string),
+  titleTemplate?: string | ((titleChunk: string) => string) | null,
 
   htmlAttrs?: AttributeProperty
   headAttrs?: AttributeProperty