Allow use of NULL to remove any previously set source notes, footnotes, and spanner column labels

[rich-iannone]

This will allow NULL to be valid input in tab_source_note() and tab_footnote(). In these, NULL isn't already used to signify a missing option, and, this change will bring the behavior closer to that of tab_header().

Fixes: #790

[jcheng5]

After discussing with @rich-iannone--I don't like this approach with functions where repeated calls result in additive effects (i.e. calling tab_footnote multiple times on a cell results in multiple footnotes, not just the most recent footnote). It's surprising to me that if you pass in a NULL, suddenly it affects all the previous calls as well. I'd prefer a tab_clear_footnote or something--or maybe tab_footnote(replace = TRUE)?

[rich-iannone]

Closing this in favor of the better approach afforded by #1073.