Fix null and empty object comparison

[jenting]

Fixes #273

We found that if the VaultSecret without label, the corresponding k8s secret keeps updating because the two secrets labels are not the same. It's because existing k8s secret labels is null, however, the k8s secret labels generated from newSecretForCR is an empty object.

The generated k8s secret labels and annotations are directly inherited from the VaultSecret CR, so it does not require to loop it and copy it again.

Also, we fix the go-staticcheck warnings that

1. should use time.Until instead of t.Sub(time.Now())
2. error strings should not be capitalized

[jenting]

@ricoberger PTAL when you're available, thank you 😊

[ricoberger]

Thanks for your fix @jenting 🙂