[clang] [clang][analyzer] Print empty per-EP metrics as empty CSV cells, fix missing PathRunningTime metric (PR #162839)

[Arseniy Zaostrovnykh via cfe-commits]

necto wrote:

FYI, I used ClaudeCode to write the example checker that exercises an unsigned explicitly-set statistic.

This is a second patch out of three, the first one (#162817) removing the unneeded boolean statistic type, and the final one will add SyntaxRunningTime similar to how PathRunningTime is recorded now.

https://github.com/llvm/llvm-project/pull/162839