fix(device_info_plus): device memory null error on Safari and Firefox

[bhaswanth-isani]

Description

Fixes type 'Null' is not a subtype of type 'double' for deviceMemory in webBrowserInfo on Safari and Firefox

Related Issues

• Fix [Bug]: DeviceInfoPlugin.webBrowserInfo fails locally on Safari #3391
• Related: Unexpected 'null' in non-nullable properties of the Navigator class dart-lang/web#326

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I titled the PR using Conventional Commits.
• I did not modify the CHANGELOG.md nor the plugin version in pubspec.yaml files.
• All existing and new tests are passing.
• The analyzer (flutter analyze) does not report any problems on my PR.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change (please indicate that with a ! in the title as explained in Conventional Commits).
• No, this is not a breaking change.

[miquelbeltran]

I did: dart-lang/web#326

[miquelbeltran]

I added a link to the dart-lang/web report, as well as changed the return value to null instead of 0.0, as the device_info_plus field supports nullable values.

[miquelbeltran]

I completely re-wrote the PR 😄 but no worries you will still be credited when merged.

[bhaswanth-isani]

It's not a problem. Thanks.