Add precondition for var.container_app_environment_internal_load_balancer_enabled

[lonegunmanb]

Describe your changes

It looks like the azurerm provider has introduced a new constriction on internal_load_balancer_enabled, once it has been set no matter the value, you must set infrastructure_subnet_id too or an error will be thrown. I've opened a pr in provider's repo for this issue, this pr changed the variable's default value to null to avoid the error caused by default values.

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Update: since #39 has already updated the variable's default value, this pr only add a precondition block for this variable to provide a better user experience.

Issue number

#32 #40

Checklist before requesting a review

• The pr title can be used to describe what this pr did in CHANGELOG.md file
• I have executed pre-commit on my machine
• I have passed pr-check on my machine

Thanks for your cooperation!

[jiaweitao001]

🚢