Fix tax year dropdown edge case

Occurs when the prior year filing contains no grants, but other years DO have grants.

Example profile
https://www.grantmakers.io/profiles/v0/010456596-alex-tanous-foundation-for-scientific-research/

Offending code
`{% if page.filings[1] and page.grants_facet_tax_year[1].count > 0 %}`

https://github.com/grantmakers/profiles/blob/source/_layouts/profile.html#L60