[voice_assistant] Bugfix: Fix crash on start

[kahrendt]

What does this implement/fix?

Adds nullptr initializations for the buffers, as it is causing issues when using the constructor instead of setup from PR #7630. Additionally, it adds some missing checks that test whether the speaker is defined or not (this will be necessary for a media player that outputs the audio through a speaker component)

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Code quality improvements to existing code or addition of tests
• Other

Related issue or feature (if applicable):

not applicable

Pull request in esphome-docs with documentation (if applicable):

not applicable

Test Environment

• ESP32
• ESP32 IDF
• ESP8266
• RP2040
• BK72xx
• RTL87xx

Example entry for config.yaml:

# Example config.yaml

external_components:
  - source: github://pr#7662
    components: [voice_assistant]

Checklist:

• The code change is tested and works locally.
• Tests have been added to verify that the new code works (under tests/ folder).

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in esphome-docs.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 54.00%. Comparing base (4d8b5ed) to head (14fedef).
Report is 1514 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #7662      +/-   ##
==========================================
+ Coverage   53.70%   54.00%   +0.29%     
==========================================
  Files          50       50              
  Lines        9408     9724     +316     
  Branches     1654     1348     -306     
==========================================
+ Hits         5053     5251     +198     
- Misses       4056     4147      +91     
- Partials      299      326      +27     

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[probot-esphome]

Hey there @jesserockz, mind taking a look at this pull request as it has been labeled with an integration (voice_assistant) you are listed as a code owner for? Thanks!
_{^{(message by CodeOwnersMention)}}