fix: default boolean false is not accepted [fixes #240]

[marina-mosti]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/formvuelate/formvuelate/blob/main/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

Closes #240
Adds the ability to use boolean false as a default value for model.
Refactors the check from a general falsey to just evaluating if the property exists in the item.

[codecov-commenter]

Codecov Report

Merging #241 (f56fefe) into main (5f368b3) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #241   +/-   ##
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  Coverage   94.87%   94.87%           
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  Files           7        7           
  Lines         195      195           
  Branches       49       49           
=======================================
  Hits          185      185           
  Misses          8        8           
  Partials        2        2           

[logaretm]

Nice!