navigation: Unset the narrative text when the icon changes

[mmind]

Navigation narrative commonly comes only at specific intervals and at least OSMand does update the direction icon and countdown-distance way earlier.

This can lead to situations when the old narrative is displayed to a mismatched direction icon.

So after the icon changes, assume the narrative is not applicable anymore.

The flag-update in the code also happens before possibly updating the narrative, so if a new narrative text is send within the same update, this will happen just normally.

[github-actions[bot]]

Build size and comparison to main:

Section	Size	Difference
text	375860B	16B
data	948B	0B
bss	22536B	0B

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