RegisterWidget: Fix crash when right-clicking when there is no selected cell

[Pokechu22]

Before, Dolphin would crash if you right-clicked in the blank space on the bottom-right, or if you right-clicked outside of the table without having first clicked on a register. Now, the context menu properly appears, with only "update" listed (I'm not entirely sure what that does, though).

Note that right-clicking outside of the table will use the currently selected register's context menu (instead of the update-only one); this behavior existed beforehand.

I also did a quick check of other uses of customContextMenuRequested, and they all seem to check that a selection exists (or else don't depend on having a selection at all), though I wasn't too deep in this checking (as long as it looked like an if-statement existed, I decided it was probably good).

[iwubcode]

Not entirely sure what the RegisterWidget is/does but looking at the previous code it is pretty obvious how it can crash and this has been fixed. LGTM

[Pokechu22]

Select View → Registers (with the debugging UI enabled). RegisterWidget shows the values of the PowerPC registers (well, most of them, at least).

[iwubcode]

Confirmed behavior by comparing latest master and this PR. Fixes the crash