fix(@ngtools/webpack): fix error in compiler CLI when i18nFormat is undefined. (#6617)

Author: rayros

packages/@ngtools/webpack/src/extract_i18n_plugin.ts

@@ -175,7 +175,7 @@ export class ExtractI18nPlugin implements Tapable {
           program: this._program,
           host: this._compilerHost,
           angularCompilerOptions: this._angularCompilerOptions,
-          i18nFormat: this._i18nFormat,
+          i18nFormat: this._i18nFormat || '',
           locale: this._locale,
           outFile: this._outFile,