Update logic added by commit 099934e313 (Add generator expression
support to static code analysis hooks, 2023-03-24, v3.27.0-rc1~261^2),
and preserved by commit 993dde925f (TargetGenerator: Factor out
generation of code check rules, 2023-05-12, v3.27.0-rc1~84^2~2), to
handle a generator expression that evaluates to the empty string.
If `<LANG>_CPPCHECK`, `<LANG>_CPPLINT`, `<LANG>_CLANG_TIDY`, or
`<LANG>_INCLUDE_WHAT_YOU_USE` are empty after evaluating generator
expressions, do not run their lints.
Fixes: #25265
80df7b1745