settings_org: Fix save-discard not hidden for "Other settings" section.
settings_org: Fix save-discard not hidden for "Other settings" section.

When resetting a setting in "Other settings" section to its original
value, the save-discard widget for the section is not hidden. This can
be reproduced when no language is set for code-block default language
setting. This was because check_property_changed incorrectly returned
true as the original value for code-block language setting was null
but the widget element returned empty string.

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