Code clean-up: prompt template string elements repeated across code locations

[dxtrous]

Is your feature request related to a problem? Please describe.
The following lines of code:
https://github.com/pathwaycom/pathway/blob/main/python/pathway/xpacks/llm/prompts.py#L121
https://github.com/pathwaycom/pathway/blob/main/python/pathway/xpacks/llm/question_answering.py#L15
https://github.com/pathwaycom/pathway/blob/main/python/pathway/xpacks/llm/prompts.py#L35
define string constants representing no-answer-found in separate ways. I am wondering if there is a purpose in having a default argument value which is overridden by its primary wrapper.

Describe the solution you'd like
This could merit clean-up.

Describe alternatives you've considered
Leave as is.

Additional context
N/A