Refactor OTP tests and cleanup code

- Removed unnecessary blank lines in otp_test.go and otp.go files for improved readability.
- Ensured consistency in test assertions for alphanumeric code generation and default behavior with zero values.
- Streamlined the benchmark test for alphanumeric code generation, enhancing performance measurement clarity.

utils/otp/otp.go

@@ -119,4 +119,3 @@ func generateRandomString(length int, charset string) string {
 
 	return string(result)
 }
-

utils/otp/otp_test.go

@@ -226,4 +226,3 @@ func BenchmarkGenerateAlphanumeric(b *testing.B) {
 		Generate(option)
 	}
 }
-