Revert "[camera_platform_interface] Migrate to null safety (#3497)"

[mvanbeusekom]

This reverts commit 545c97f.

After merging PR #3497 I received some valuable feedback from @ditman which I would really like to include in this first NNBD version. So I created this revert PR to rollback the changes in master and will make sure a new PR is created to re-land these changes including the feedback from @ditman.

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