processor: Fix a few more potential nullpointers during json export
processor: Fix a few more potential nullpointers during json export

Bug: 130786269
Change-Id: I378f1adcee871fd11cdd5b6eaa3a5af0a744caa7
google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto