Update assertions on freeDisk on iOS

[gingerbenw]

Goal

Add null check for freeDisk on iOS to existing tests - this value is no longer automatically set on iOS.

This could be manually set and asserted against in the scenario, i.e.:

[Bugsnag.instance.configuration.device setValue:@(1234567) forKey:@"freeDisk"];

but does not feel like a useful test.

[github-actions]

@bugsnag/browser bundle size diff

	Minified	Minfied + Gzipped
Before	51.76 kB	15.30 kB
After	51.76 kB	15.30 kB
±	No change	No change

code coverage diff

Coverage values did not change👌.

Total:

Lines	Branches	Functions	Statements
0%(+0%)	0%(+0%)	0%(+0%)	0%(+0%)

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