oeqa/runtime/ptest: Make returning no test results a failure

oeqa/runtime/ptest: Make returning no test results a failure

Ensure that even if a ptests results section is empty, the log parser adds that
empty section. Then ensure that empty sections trigger warnings.

This means if a ptest suddently stops returning any results, we notice and see
warnings about it. This has gone unnoticed on the autobuilder far too many times
so is very much worth highlighting as a regression. We shouldn't have empty ptests.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>