[fix](sql block) throw npe when use Nereids explain or fallback

[morrySnow]

Proposed changes

Issue Number: close #xxx

Further comments

If this is a relatively large or complex change, kick off the discussion at dev@doris.apache.org by explaining why you chose the solution you did and what alternatives you considered, etc...

[morrySnow]

run buildall

[doris-robot]

(From new machine)TeamCity pipeline, clickbench performance test result:
the sum of best hot time: 46.48 seconds
stream load tsv: 580 seconds loaded 74807831229 Bytes, about 123 MB/s
stream load json: 21 seconds loaded 2358488459 Bytes, about 107 MB/s
stream load orc: 64 seconds loaded 1101869774 Bytes, about 16 MB/s
stream load parquet: 30 seconds loaded 861443392 Bytes, about 27 MB/s
insert into select: 28.8 seconds inserted 10000000 Rows, about 347K ops/s
storage size: 17162335681 Bytes

[morrySnow]

run buildall

[doris-robot]

(From new machine)TeamCity pipeline, clickbench performance test result:
the sum of best hot time: 45.35 seconds
stream load tsv: 585 seconds loaded 74807831229 Bytes, about 121 MB/s
stream load json: 21 seconds loaded 2358488459 Bytes, about 107 MB/s
stream load orc: 64 seconds loaded 1101869774 Bytes, about 16 MB/s
stream load parquet: 29 seconds loaded 861443392 Bytes, about 28 MB/s
insert into select: 28.6 seconds inserted 10000000 Rows, about 349K ops/s
storage size: 17162399024 Bytes

[github-actions]

PR approved by at least one committer and no changes requested.

[github-actions]

PR approved by anyone and no changes requested.