Trailing escape ('ab\') is disallowed in SQL, in standardese:
"If an escape character is specified, then [...] If there is not a
partitioning of the string PVC into substrings such that each substring
has length 1 (one) or 2, no substring of length 1 (one) is the escape
character ECV, and each substring of length 2 is the escape character
ECV followed by either the escape character ECV, an <underscore>
character, or the <percent> character, then an exception condition is
raised: data exception - invalid escape sequence."
I first thought this is checked already higher up in the stack, at least
for const needles, as single trailing backslashes ('ab\') are rejected,
but then I realized that ClickHouse quotes by default. I.e., double
trailing backslashes ('ab\\') are not rejected but when interpreted as
LIKE needle ('ab\') they should.
