Fix cdktf synth --hcl fails to render false(bool), 0(int), ""(empty string)

[XIAOLONGLII]

Added test for variable bool and string default value.

Related issue

Fixes #3621

Description

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Checklist

• I have updated the PR title to match CDKTF's style guide
• I have run the linter on my code locally
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation if applicable
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works if applicable
• New and existing unit tests pass locally with my changes

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