Backends: Allegro: Don't call AddInputCharacter if the pressed key has no character.

[Espyo]

• Commit ef13d95 moved the logic of checking for invalid Unicode codes from each backend to Dear ImGui's AddInputCharacter function proper.
• Commit c8ea0a0 then removed that logic, replacing it with something that inserts a question mark-like character in the case of an invalid Unicode code.
• As a result, after these two commits, if an Allegro keypress has no associated character (arrow keys, End, Page Up, etc.), it will send zero with nobody to filter it out.
• With this fix, no-character inputs won't even call AddInputCharacter to begin with.
• Tested with arrow keys, Page Up, etc., tested with regular character keys, and tested with some Unicode symbols directly from the keyboard, like €.

[ocornut]

Thanks @Espyo for the thoughtful commit and explanation.

Looking at c8ea0a0 again, I feel like we should add a != 0 test in the lower-level functions AddInputCharacter(). I guess we were focusing on the "other" invalid inputs but since lows of back-ends have been passing value to AddInputCharacter() filtered it makes sense to add the test back as ef13d95 intended.

I don't mind also leaving the if != 0 in Allegro back-end because that event can query both types of data.