Fix implicit result column name/alias for SELECT item literals with NULL bytes

[JanJakes]

Fixes SELECT queries that have string literals with NULL bytes in them and no column alias.

E.g.:

SELECT `a<null-byte>b`;

-- result column name should be: 'a'

This is because in MySQL:

• There can’t be \0 bytes in identifiers (like column names). The lexer will reject that.
• But string literals can have them.
• And there is a way to make a string literal “identifier-like” (implicit alias = the result “column name”): SELECT 'abc\0xyz' The result column name will be: abc

[adamziel]

Does this matter in more places than just translate_select_item?

[JanJakes]

@adamziel I will check that a bit deeper in a follow-up PR. There is the case where you type this explicitly (SELECT 1 AS 'a\0b'), and we need a follow-up to tackle that. Then, I think there is no other way where a string literal gets "converted" to an alias, but—I found some inconsistencies in how we process string literals vs. quoted identifiers and want to improve that too. We seem to treat them the same, but they shouldn't. It might be an interesting edge case bug going down to the lexer.

[adamziel]

LGTM and solves a problem so looks good for merging. We may want to follow up with a more general solution if that matters beyond the SELECT columns list.