fix: pptx line break and space handling

[mawi12345]

The PR improves the handling of LineBreaks and spaces in the pptx backend. LineBreaks were ignored before the PR and are now replaced by a space. Empty TextRuns were also ignored which leads to problems if there is only one empty TextRun between two words.

The test file powerpoint_bad_text.pptx currently outputs:

# X-LibraryThe fully customisable and copyright-freestandardcontenttemplatecollectionexclusivelyforourcustomers

With the PR:

# X-Library The fully customisable and copyright-free standard content template collection exclusively for our customers

Checklist:

• [ ] Documentation has been updated, if necessary.
• [ ] Examples have been added, if necessary.
• [x] Tests have been added, if necessary.

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Codecov Report

Attention: Patch coverage is 96.42857% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
docling/backend/mspowerpoint_backend.py	96.42%	1 Missing :warning:

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