input-method-v1: Fix bug regarding the modifier handling

modifiers request by the input method is supposed to send the raw
modifiers based on the keymap of the keyboard grabbed as result of the
grab_keyboard request. If input method client is not using the keysym
functionality it can decide to not send out the modifiers_map, since it
is already known to compositor as part of keymap event sent by it.

While at it also guard against empty modifiers_map, if this happens
ideally compositor should handle that information based on the keymap
sent out using grab_keyboard function, but since currently, we do not
have the grab_keyboard implemented in here, send out the NoModifier if
that happens.