Fixes error found in rolling to google.

```
[☠] Android toolchain - develop for Android devices (the doctor check crashed)
    ✗ Due to an error, the doctor check did not complete. If the error message
      below is not helpful, please let us know about this issue at
      https://github.com/flutter/flutter/issues.
    ✗ type 'Null' is not a subtype of type 'String' of 'executable'
    • #0      LocalProcessManager.canRun
      (package:process/src/interface/local_process_manager.dart:124)
      flutter#1      getEmulatorVersion
      (package:flutter_tools/src/android/android_workflow.dart:64)
      flutter#2      AndroidValidator.validateImpl
      (package:flutter_tools/src/android/android_workflow.dart:200)
      flutter#3      DoctorValidator.validate
      (package:flutter_tools/src/doctor_validator.dart:58)
      flutter#4      Doctor.startValidatorTasks.<anonymous closure>
      (package:flutter_tools/src/doctor.dart:244)
      flutter#5      asyncGuard.<anonymous closure>
      (package:flutter_tools/src/base/async_guard.dart:109)
      flutter#6      _rootRun (dart:async/zone.dart:1525)
      flutter#7      _CustomZone.run (dart:async/zone.dart:1422)
      flutter#8      _runZoned (dart:async/zone.dart:2033)
      flutter#9      runZonedGuarded (dart:async/zone.dart:2019)
      flutter#10     runZoned (dart:async/zone.dart:1952)
      flutter#11     asyncGuard (package:flutter_tools/src/base/async_guard.dart:106)
      flutter#12     Doctor.startValidatorTasks
      (package:flutter_tools/src/doctor.dart:234)
      flutter#13     Doctor.diagnose (package:flutter_tools/src/doctor.dart:372)
      flutter#14     DoctorCommand.runCommand
      (package:flutter_tools/src/commands/doctor.dart:59)
      flutter#15     FlutterCommand.verifyThenRunCommand
      (package:flutter_tools/src/runner/flutter_command.dart:1897)
      <asynchronous suspension>
      flutter#16     FlutterCommand.run.<anonymous closure>
      (package:flutter_tools/src/runner/flutter_command.dart:1551)
      <asynchronous suspension>
      flutter#17     AppContext.run.<anonymous closure>
      (package:flutter_tools/src/base/context.dart:154)
      <asynchronous suspension>
      flutter#18     CommandRunner.runCommand (package:args/command_runner.dart:212)
      <asynchronous suspension>
      flutter#19     FlutterCommandRunner.runCommand.<anonymous closure>
      (package:flutter_tools/src/runner/flutter_command_runner.dart:501)
      <asynchronous suspension>
      flutter#20     AppContext.run.<anonymous closure>
      (package:flutter_tools/src/base/context.dart:154)
      <asynchronous suspension>
      flutter#21     FlutterCommandRunner.runCommand
      (package:flutter_tools/src/runner/flutter_command_runner.dart:438)
      <asynchronous suspension>
      flutter#22     run.<anonymous closure>.<anonymous closure>
      (package:flutter_tools/runner.dart:98)
      <asynchronous suspension>
      flutter#23     AppContext.run.<anonymous closure>
      (package:flutter_tools/src/base/context.dart:154)
      <asynchronous suspension>
      flutter#24     AppContext.run.<anonymous closure>
      (package:flutter_tools/src/base/context.dart:154)
      <asynchronous suspension>
      flutter#25     run (package:mobile.flutter.cli/flutter_tools.dart:106)
      <asynchronous suspension>
      flutter#26     main (google3:///mobile/flutter/cli/bin/cli_usage_aot.dart:4)
```

## Pre-launch Checklist

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

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

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