Fix system ini handling of invalid values in zai config

[bwoebi]

Description

Also handle the special case of the empty string in the profiling log level parser because of an unquoted "off" in ini becoming an empty string.

Readiness checklist

• (only for Members) Changelog has been added to the release document.
• Tests added for this feature/bug.

Reviewer checklist

• Appropriate labels assigned.
• Milestone is set.
• Changelog has been added to the release document. For community contributors the reviewer is in charge of this task.

[morrisonlevi]

For historical reference, this came up specifically when testing:

; 98-ddtrace.ini
datadog.profiling.log_level=true

; 99-whatever.ini
datadog.profiling.log_level=off

And I got a warning in the profiler:

[2023-03-20T22:54:55.430860Z WARN  datadog_php_profiling::config] zval of type 15 encountered when calling config::profiling_log_level(), expected type int (4)

Thanks, @bwoebi, for fixing this so quickly.