Reject empty TRACE in parse_trace_string
Reject empty TRACE in parse_trace_string

Summary:
A poor person who did `export TRACE=""` will see a parse exception in
parse_trace_string. A quick fix that rejects empty TRACE string.

So the correct way to unset TRACE is `unset TRACE`.

Reviewed By: int3

Differential Revision: D5281812

fbshipit-source-id: 2fcca79d6e803908bec64ebee9eb6399001852da

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