fix(ui): handle undefined dates in Workflows List filter

[agilgur5]

Another fix for #9696

Motivation

• when date filters are removed, they are set to undefined
  • the previous logic did not handle this correctly, comparing undefined with Dates
    • comparing to undefined always returns false
  • new logic only compares when Dates are set

Modifications

• only compare the Dates when they are actually set, otherwise return true

Verification

Can see in the below screenshot (in React DevTools) that minStartedAt was set to undefined after I hit the "x" button on the date filter:

Made sure that undefined did in fact act weirdly, and here's results from browser console REPL:

undefined < new Date()
// false
undefined > new Date()
// false
new Date() > undefined
// false
new Date() < undefined
// false

Tested manually in the UI and made sure it works as expected