Unable to save existing secret without re-entering plaintext

### Environment
* Python version: 3.6.9
* NetBox version: 2.9.10

### Steps to Reproduce
1. Create a secret
2. Edit the secret and change its role or name without specifying a new plaintext value
3. Save the secret

### Expected Behavior
The secret should be saved with its new attributes.

### Observed Behavior
An exception is raised: `Must unlock or set plaintext before locking.`