fix(api): resolve type errors in tag service

apps/api/src/services/tag/tag.service.ts

@@ -19,7 +19,7 @@ export class TagService {
 
   public async getTag(
     tagWhereUniqueInput: Prisma.TagWhereUniqueInput
-  ): Promise<Tag> {
+  ): Promise<Tag | null> {
     return this.prismaService.tag.findUnique({
       where: tagWhereUniqueInput
     });