AuthService.randomString: Throw Error instead of returning null

A string with a negative length is invalid, so we should throw here
instead of complicating the type with a possible null return value.

Signed-off-by: David Mehren <git@herrmehren.de>

src/auth/auth.service.spec.ts

@@ -319,4 +319,10 @@ describe('AuthService', () => {
       );
     });
   });
+  describe('randomString', () => {
+    it('throws on invalid lenght parameter', () => {
+      expect(() => service.randomString(0)).toThrow();
+      expect(() => service.randomString(-1)).toThrow();
+    });
+  });
 });

src/auth/auth.service.ts

@@ -64,7 +64,7 @@ export class AuthService {
 
   randomString(length: number): Buffer {
     if (length <= 0) {
-      return null;
+      throw new Error('randomString cannot have a length < 1');
     }
     return randomBytes(length);
   }