Previous change in DropDownMenu #162380 forced the menu width to be constrained between the width or anchorWidth and the maximumSize width menuStyle, but giving a minimum of zero when maximumSize was not enforced or null. This prevents width or anchorWidth to be used at all as the minimum will be zero always if maximumSize is null

Now this sets the default value back to width or anchorWidth if maximumSize is null, keeping consistency of the previous configuration while using the fix of maximumSize when provided.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

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