Fix isDense default for DropdownButtonFormField - #47160

Description

Fixes a regression to the vertical size of DropdownButtonFormField caused by a change in the default value of isDense:

After Upgrading to Flutter v1.12.13+hotfix.5

Before Upgrading to v1.12.13+hotfix.5

Related Issues

Fixes #46844

Tests

I added the following tests:

• A test to ensure that DropdownButtonFormField.isDense is true by default.

Checklist

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Breaking Change

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