[Teletext] Only override the language code from header if it is actua…

[enen92]

…lly valid

Description

In our teletext implementation we always override the national subset which might have been received from a Packet X/28 (see https://www.etsi.org/deliver/etsi_en/300700_300799/300706/01.02.01_60/en_300706v010201p.pdf table 33) with the page header country code. The issue is that even if the header doesn't have a valid subset (0x00 countrycode) we set it anyway overriding any valid value previously set.

Motivation and context

Fix #23882

How has this been tested?

Runtime tested against the provided sample as well as my other test samples (and actual tv channels):

What is the effect on users?

Some charsets were incorrectly setcausing visual glitches and unknown characters in the teletext browser.

Screenshots (if appropriate):

Before:

Now:

Other tested samples with different charsets:
(pt)

(spanish)

(uk)

(russian)

Types of change

• Bug fix (non-breaking change which fixes an issue)
• Clean up (non-breaking change which removes non-working, unmaintained functionality)
• Improvement (non-breaking change which improves existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that will cause existing functionality to change)
• Cosmetic change (non-breaking change that doesn't touch code)
• Student submission (PR was done for educational purposes and will be treated as such)
• None of the above (please explain below)

[ninja-]

Thanks! I would never expect the issue to be that far away from Teletext.cpp...

[ksooo]

LGTM.