fix: handle null configurations key in reconfigure dart config

[Harshit-0413]

Fixes #353

Problem

When running flutterfire configure twice and choosing to reuse the
existing firebase.json, the command crashes with:

Exception: type 'Null' is not a subtype of type 'Map<String, dynamic>' in type cast

This happens in _writeDartConfigurationFile in reconfigure.dart
where map[kConfigurations] is cast directly without a null check.
If the existing firebase.json was written by an older CLI version
that didn't include the configurations key, the cast throws.

Fix

Cast map[kConfigurations] as nullable Map<String, dynamic>? and
return early if null, skipping that entry gracefully.

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[gemini-code-assist]

Code Review

This pull request updates the reconfigure.dart file to handle cases where the configurations map might be null. It introduces a null-safe cast and a check that returns an empty list if the configurations are missing, preventing potential runtime errors. I have no feedback to provide.