input/csv: Fix a false negative after successful import

input/csv: Fix a false negative after successful import

The input module runs receive() and end() invocations which end up
calling process_buffer(). It's perfectly legal to call the process
routine with an empty accumulation buffer, especially when the process
routine was called from end().

This fixes a condition where PulseView raised a fatal error at the end
of a completed successful import.

Reported-By: Sergey Alirzaev <redacted>