sessionctx/variable: fix panic when set variable=''

[crazycs520]

What problem does this PR solve?

fix panic when execute below sql:

set @@global.max_user_connections='';

What is changed and how it works?

fix checkUInt64SystemVar bug.

Check List

Tests

• Unit test

Code changes

Side effects

Related changes

• Need to cherry-pick to the release branch

[crazycs520]

/run-all-tests

[codecov-io]

Codecov Report

Merging #9533 into master will decrease coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #9533      +/-   ##
==========================================
- Coverage   67.38%   67.38%   -0.01%     
==========================================
  Files         374      374              
  Lines       78738    78740       +2     
==========================================
- Hits        53058    53057       -1     
- Misses      20905    20907       +2     
- Partials     4775     4776       +1

Impacted Files	Coverage Δ
sessionctx/variable/varsutil.go	25.3% <0%> (-0.16%)	⬇️
store/tikv/scan.go	73.94% <0%> (-3.37%)	⬇️
store/tikv/lock_resolver.go	41.7% <0%> (-0.95%)	⬇️
executor/join.go	79.58% <0%> (+0.51%)	⬆️
executor/index_lookup_join.go	78.05% <0%> (+0.62%)	⬆️
expression/schema.go	94.53% <0%> (+0.78%)	⬆️

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[alivxxx]

LGTM

[eurekaka]

LGTM