fix(realm/Results): provide fallback condition

[guillemsarda]

What, How & Why?

Small change in the getter length from Results' class.
This solves an issue that causes an app crash when upgrading from react-native: 0.71.2, realm: 11.9.0 and @realm/react: 0.4.3 to the corresponding latest versions.
The issue is solved by providing a fallback condition in the getter (i.e., when this.internal is not defined).

This closes #6954

[kraenhansen]

Thanks for your interest in solving this. We've had other users experiencing this issue and another PR trying to fix it.

It is however an invariant of the Results to have an internal value, so instead of handling the possibility of this internal going missing, the proper solution would be to figure out why this internal doesn't have a value in the first place and fix that.

This pattern of internal objects are something we're using lots of places and I don't want to merge a fix for a problem with an undetermined root cause.