fix: initialize variables to prevent potentially uninitialized usage

[dongfengweixiao]

Fixed compiler warnings about potentially uninitialized variables in handle_key_down and hkm_unregister functions by adding else clauses to set default empty string values.

${PROJECT_ROOT}/linux/flutter/ephemeral/.plugin_symlinks/hotkey_manager_linux/linux/hotkey_manager_linux_plugin.cc:41:7: error: variable 'identifier' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
${PROJECT_ROOT}/linux/flutter/ephemeral/.plugin_symlinks/hotkey_manager_linux/linux/hotkey_manager_linux_plugin.cc:112:7: error: variable 'keystring' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]