Add a missing null check for projectItem when processing recordings.

[jklein24]

This check is a couple places in this function already, but this one was missing.

[drowaudio]

Cheers, this should be fixed here: https://github.com/Tracktion/tracktion_engine/commit/6ccabdcebc4a437cfd88a32fa628cf1445036eaa

[github-actions[bot]]

Unit Test Results

       4 files   -        2         4 suites   - 2   19m 22s :stopwatch: +34s    110 tests ±       0     110 :heavy_check_mark: ±       0  0 :zzz: ±0  0 :x: ±0  3 120 runs   - 1 560  3 120 :heavy_check_mark:  - 1 560  0 :zzz: ±0  0 :x: ±0 

Results for commit 6ccabdce. ± Comparison against base commit 4a2466c6.

[github-actions[bot]]

Performance Test Results

0 files  ±0  0 suites  ±0   0s :stopwatch: ±0s 0 tests ±0  0 :heavy_check_mark: ±0  0 :zzz: ±0  0 :x: ±0 

Results for commit 6ccabdce. ± Comparison against base commit 4a2466c6.