Remove empty mitigation_issue from feat_saf_dfa__persistency__execution_blocking

### Description

## Description

The `feat_saf_dfa__persistency__execution_blocking` need has an empty
`mitigation_issue` attribute (`""`). The schema validation check requires this field,
if present, to match `^https://github.com/.*$`.

### Analysis results

This was not caught before because the existing Python-based post-build checks
did not flag empty strings. With the introduction of sphinx-needs 6 schema
validation (generated from the metamodel in `score-docs-as-code`), schemas are
now evaluated at parse time on all needs -- including those pulled from external
repositories. The schema correctly rejects the empty string as a pattern
mismatch.

### Error

ERROR: Need 'feat_saf_dfa__persistency__execution_blocking' has schema violations:
Field: mitigation_issue
Schema message: "" does not match "^https://github.com/.*$"

### Solution

### Fix

Remove the empty `mitigation_issue` attribute. If no mitigation issue exists,
the field should be absent rather than set to an empty string.

### Error Occurrence Rate

None

### How to reproduce

_No response_

### Supporting Information

_No response_

### Classification

minor

### Affected Version

pre-0.5

### Expected Closure Version

0.5

### Category

- [ ] Safety Related
- [ ] Security Related

### ASIL classification

_No response_