Bug 408075

SUMMARY
Okular binds the alt+shift+left/right shortcuts to the 'back' and 'forward' items by default. If the history is exhausted (i.e. we navigate back/forward till there are no more locations left), okular starts moving to the next/previous page, acting as if we have pressed just the left/right arrow key.

STEPS TO REPRODUCE
1. Open a PDF in Okular.
2. Add the back and forward buttons to the toolbar (this is just to illustrate the issue)
3. Note that the 'forward' button is greyed out.
4. Press Alt+Shift+Right (forward). 
5. Note that Okular moves forward one page.
6. Now scroll up and down to populate the history.
7. Press Alt+Shift+Left until the history of previous locations is exhausted. At this point, the 'back' button will be greyed out and unclickable.
8. Press Alt+Shift+Left
8. Note that this causes Okular to move one page back (as if we had just pressed the left arrow key)

OBSERVED RESULT
Okular flips pages when there is nowhere to go to in the history.

EXPECTED RESULT
Okular should do nothing if history is exhausted (i.e. after steps 8 and 4).

SOFTWARE/OS VERSIONS
Linux distro: Arch Linux
KDE Plasma Version: 5.15.90
KDE Frameworks Version: 5.58.0
Qt Version: 5.13.0

ADDITIONAL INFORMATION
This is reproducible only if the shortcut contains an arrow key. If we bind something like ctrl+9 to 'forward', Okular shows the expected behaviour. Prima facie, it looks like Okular doesn't correctly handle the keypresses corresponding to a shortcut if the shortcut corresponds to an action that cannot be performed (like going 'forward' when we are at the most recently veiwed location).