Histogram function may crash the whole nebula node

[shawncao]

Some error log captured when a nebula node dies due to a hist() call in production

I0128 19:12:14.229609 20261 Dsl.cpp:354] Nodes to execute the query: 1
I0128 19:12:14.230307 20261 BlockManager.cpp:147] Fetch blcoks 1009 / 1009 for table cdn_requests in window [1611838876, 1611860964].
I0128 19:12:14.230335 20261 NodeExecutor.cpp:84] Processing total blocks: 1009
F0128 19:12:14.232291 36092 Histogram-inl.h:34] Check failed: bucketSize_ > ValueType(0) (0 vs. 0)
*** Check failure stack trace: ***
*** Aborted at 1611861134 (Unix time, try 'date -d @1611861134') ***
*** Signal 6 (SIGABRT) (0x3e800000006) received by PID 6 (pthread TID 0x7fe8c5ec2700) (linux TID 36092) (maybe from PID 6, UID 1000) (code: -6), stack trace: ***
(error retrieving stack trace)

[shawncao]

Bucket size is 0, I guess because min/max are both 0 for some column if they have single value 0. So we need to handle this case, maybe by returning a single bucket histogram JSON directly from query planning phase?