Editing default Prompt clears prompt, even it looks like it is still there

[bmihelac]

When editing default prompts, such as changing the label of the "AI Correction" prompt, the Prompt.prompt field's value changes from NULL to an empty string. This causes an empty prompt_value, resulting in incorrect outputs when used in the editor.

To reproduce:

1. edit default prompt or just open and save it
2. use it in the editor

Proposed Fixes:

1. Prompt.prompt should not be nullable; its default should be an empty string
2. Prompt.prompt_value should test truthiness instead of explicitly checking for None

I can submit a pull request if needed.