Make sure prices are actual pairs when updating

When passing a pair to addPricePair() in Update Stock and Currency Prices dialog, actually make sure that both elements are non-empty in order to avoid an undefined behavior and SEGFAULTS.

One thing this change doesn't explain (or fix) is the reason why are such half-empty pairs even possible and present in my .kmy file, as explained in the bug report. However, this deserves its own separate investigation and would, if at all, result in a different bug report. This PR is to fix the SEGFAULTS resulting from the code trying to process such pairs, and that only.

BUG: 422561 FIXED-IN: 5.1.2