esphome::optional fixes

[twasilczyk]

What does this implement/fix?

esphome::optional has two problems that prevent its use in code generation

• esphome::nullopt isn't available from cpp_types list (so code generator can't refer to it like to nullptr)
• you can't initialize optional like you would T (so code generator can't just pass e.g. "some string" to esphome::optional<std::string>) - that's the (8) constructor from this list

By the way, I have a question: why can't we just use std::optional?

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)
• Other

Test Environment

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

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 54.15%. Comparing base (4d8b5ed) to head (ef7cfac).
Report is 514 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #6681      +/-   ##
==========================================
+ Coverage   53.70%   54.15%   +0.44%     
==========================================
  Files          50       50              
  Lines        9408     9576     +168     
  Branches     1654     1687      +33     
==========================================
+ Hits         5053     5186     +133     
- Misses       4056     4067      +11     
- Partials      299      323      +24     

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[kbx81]

By the way, I have a question: why can't we just use std::optional?

There's been some discussion about this on Discord a few times. I don't recall the details, but I don't think there was any specific opposition to using std::optional. If you want to look into migrating to it (perhaps that would be a better use of this PR), I don't think we'd be opposed. That said, some of the toolchains may be a bit dated, so if you chase it, be sure to test very, very thoroughly.

[twasilczyk]

Ouch. I didn't mean to make a huge change out of this (I did migrations like that at work, they can be nasty). And I already aim at other large change to work on (esphome/feature-requests#1568)...

Let me try to fix this one for now (I see some tests are not passing) and maybe I'll pick up migration in the future.