When building
https://github.com/flutter/samples/tree/main/add_to_app/multiple_flutters,
the `[CP-User] Run Flutter Build multiple_flutters_module Script` step
failed with:
```
Showing Recent Messages
[   +3 ms] Improperly formatted define flag: 
[        ] 
           #0      throwToolExit (package:flutter_tools/src/base/common.dart:34:3)
           flutter#1      AssembleCommand._parseDefines (package:flutter_tools/src/commands/assemble.dart:279:9)
```

the command was:
```
 /Users/<User>/flutter/bin/flutter --verbose assemble --no-version-check --output=/Users/<User>/Library/Developer/Xcode/DerivedData/MultipleFluttersIos-<Hash>/Build/Products/Debug-iphonesimulator/ -dTargetPlatform=ios -dTargetFile=lib/main.dart -dBuildMode=debug -dConfiguration=Debug -dIosArchs=arm64 -dSdkRoot=/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator26.0.sdk -dSplitDebugInfo= -dTreeShakeIcons=false -dTrackWidgetCreation=true -dDartObfuscation=false -dAction=build -dFrontendServerStarterPath= --ExtraGenSnapshotOptions= --DartDefines= --ExtraFrontEndOptions= -dSrcRoot=/Users/<User>/flutter_debug/samples/add_to_app/multiple_flutters/multiple_flutters_ios -dTargetDeviceOSVersion=26.0 -dCodesignIdentity=- debug_ios_bundle_flutter_assets
```

the empty `--DartDefines=` comes from
https://github.com/flutter/flutter/blob/68cddc9bf5c449d5a55fe48ebb9b446ef76ae6b0/packages/flutter_tools/bin/xcode_backend.dart#L660

This PR makes `_parseDefines` ignore these empty dart defines.

## 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.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

**Note**: The Flutter team is currently trialing the use of [Gemini Code
Assist for
GitHub](https://developers.google.com/gemini-code-assist/docs/review-github-code).
Comments from the `gemini-code-assist` bot should not be taken as
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useful you can update your code accordingly, but if you are unsure or
disagree with the feedback, please feel free to wait for a Flutter team
member's review for guidance on which automated comments should be
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<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md