Fix shouldn't stop on nil values when traversing the AST

[erezrokah]

This started as a bug fix the Walk function. It skips traversing constraints if Right is nil

clickhouse-sql-parser/parser/walk.go

Line 193 in 294aafb

if !Walk(n.Right, fn) {

clickhouse-sql-parser/parser/walk.go

Line 11 in 294aafb

if node == nil || !fn(node) {

This PR tries a more generic approach to prevent this bug and others by returning true if the node is nil.
I also added a test that fails without this fix

[coveralls]

Pull Request Test Coverage Report for Build 17773387987

Details

• 177 of 330 (53.64%) changed or added relevant lines in 1 file are covered.
• 162 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.4%) to 50.065%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
parser/walk.go	177	330	53.64%

Files with Coverage Reduction	New Missed Lines	%
parser/walk.go	162	41.55%

Totals
Change from base Build 17635045137:	-0.4%
Covered Lines:	7292
Relevant Lines:	14565

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💛 - Coveralls

[git-hulk]

@erezrokah Thanks for your great fix.