Empty corpus download leads to upload failure

[tysmith]

Also we should not need to run corpus merge on an empty corpus.

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Downloading corpus from gcs://guided-fuzzing-data/thunderbird-mime-decoder/corpus/ to /home/worker/workspace/queues
download_resources() -> path=corpus, total=0, downloaded=0, extracted=0, skipped=0 (0.133s)
Downloading queues from gcs://guided-fuzzing-data/thunderbird-mime-decoder/queues/ to /home/worker/workspace/queues
download_resources() -> path=queues, total=0, downloaded=0, extracted=0, skipped=0 (0.026s)
Running libFuzzer merge
*** You are running in headless mode.
INFO: Running with entropic power schedule (0xFF, 100).
INFO: Seed: 2714624801
INFO: Loaded 2 modules   (2553360 inline 8-bit counters): 14912 [0x6d6ebff58b80, 0x6d6ebff5c5c0), 2538448 [0x6d6eb9b1a150, 0x6d6eb9d85d20), 
INFO: Loaded 2 PC tables (2553360 PCs): 14912 [0x6d6ebff5c5c0,0x6d6ebff969c0), 2538448 [0x6d6eb9d85d20,0x6d6ebc441a20), 
error: Merge returned empty result, refusing to upload.
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