fix throttle average nan handling

[ssieb]

What does this implement/fix?

If the throttle_average filter receives no values during the interval, it publishes a NaN value. That can break things and doesn't really make sense. Now it won't publish a value if it didn't receive any.
I don't think this would be a breaking change, since it's very unlikely that anyone would be relying on this.
The current functionality is breaking configs.

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

Related issue or feature (if applicable): fixes

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

Test Environment

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

Example entry for config.yaml:

# Example config.yaml

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.

[probot-esphome]

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

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 53.53%. Comparing base (4d8b5ed) to head (471d49f).
Report is 18 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #6275      +/-   ##
==========================================
- Coverage   53.70%   53.53%   -0.17%     
==========================================
  Files          50       50              
  Lines        9408     9421      +13     
  Branches     1654     1652       -2     
==========================================
- Hits         5053     5044       -9     
- Misses       4056     4078      +22     
  Partials      299      299              

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