sway/input: don't pass possibly invalid modifiers pointer
sway/input: don't pass possibly invalid modifiers pointer

active_keyboard may be NULL, in which case an invalid pointer could be
passed to wlr_input_method_keyboard_grab_v2_send_modifiers. This
procedure call is unnecessary since wlroots commit 372a52ec "input
method: send modifiers in set_keyboard", so the call can simply be
removed.

Fixes #6836.

1ace/sway,1ace/sway,1ace/sway