fix(lint): fix ts-lint warning

Author: Demivan

src/fluent-vue.ts

@@ -13,7 +13,7 @@ export default class FluentVue {
   format(key: string, value?: object): string {
     const message = this.options.bundle.getMessage(key)
 
-    if (message == null || message.value == null) {
+    if (message === undefined) {
       warn(false, `Could not find translation for key [${key}]`)
       return key
     }

src/types/fluend.d.ts

@@ -28,16 +28,16 @@ declare module '@fluent/bundle' {
   class Pattern {}
 
   class MessageInfo {
-    value: Pattern | null
+    value: Pattern
     attributes: Record<string, Pattern>
   }
 
   export class FluentBundle {
     constructor(locales: string | string[], options?: FluentBundleContructorOptions)
     locales: string[]
     hasMessage(id: string): boolean
-    getMessage(id: string): MessageInfo
-    formatPattern(message: Pattern | null, args?: object, errors?: Array<string | Error>): string
+    getMessage(id: string): MessageInfo | undefined
+    formatPattern(message: Pattern, args?: object, errors?: Array<string | Error>): string
     addResource(res: FluentResource): string[]
   }
 }