[template/text] Fix lambda config

[asergunov]

What does this implement/fix?

The template text platform was not working if lambda specified. The problem is default=False in schema producing the config option even if user is not specifying it, but on validation step we checking if this option doesn't exists in config.

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

Example entry for config.yaml:

text:
  - platform: template
    mode: text
    lambda: |
      return std::string{"Hello"};

Checklist:

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

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 54.05%. Comparing base (4d8b5ed) to head (553026e).
Report is 493 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #6655      +/-   ##
==========================================
+ Coverage   53.70%   54.05%   +0.34%     
==========================================
  Files          50       50              
  Lines        9408     9554     +146     
  Branches     1654     1687      +33     
==========================================
+ Hits         5053     5164     +111     
- Misses       4056     4066      +10     
- Partials      299      324      +25     

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[esphome]

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