fix(navbar): Validate gap and padding early

[gadenbuie]

As pointed out in #761, page_navbar(padding = 24) isn't handled in the same way as layout_sidebar(padding = 24). In the second case, we pass padding through validateCssPadding(), which resovles 24 to "24px".

In page_navbar(), gap and padding are handed down to many lower-level functions until they're actually used. I think there was an assumption that one of those function would handle the validation.

For the best user experience (finding out early with a reasonable traceback), I've added early validation of these values in page_navbar() and navset_bar(). I also modified validateCssPadding() to return NULL in the base case, which makes it easier to perform the checks and matches validateCssUnit().