Compile error for voice_assistant: micro_wake_word_ not #ifdef guarded in implementation

[tetele]

The definition of the VoiceAssistant::micro_wake_word_ private property is properly guarded using #ifdef USE_MICRO_WAKE_WORD (which the micro_wake_word components sets).

However, in the implementation there are multiple places where this->micro_wake_word_ is unguarded (one, two), which causes errors at compile time if the config does not define micro_wake_word.