fix(processors.parser): Keep name of original metric if parser doesn't return one

[srebhan]

• Updated associated README.md.
• Wrote appropriate unit tests.
• Pull request title or commits are in conventional commit format

resolves #12115

With PR #11600 parsers for processors are no longer initialized with an empty metric name by default. Instead they now use the name of the parent plugin. However, processors.parser tries to use the original metric-name in case the parser does not provide an individual name (e.g. for parsers.value) for the returned metric. To do so, the processor checks for an empty name which is no longer valid as the parser's default metric name is now "parser".

This PR fixes the check for the default name to restore the old behavior.

[powersj]

Thanks for jumping on this one!

[telegraf-tiger]

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DEB	RPM	TAR GZ	ZIP
amd64.deb	aarch64.rpm	darwin_amd64.tar.gz	windows_amd64.zip
arm64.deb	armel.rpm	darwin_arm64.tar.gz	windows_i386.zip
armel.deb	armv6hl.rpm	freebsd_amd64.tar.gz
armhf.deb	i386.rpm	freebsd_armv7.tar.gz
i386.deb	ppc64le.rpm	freebsd_i386.tar.gz
mips.deb	riscv64.rpm	linux_amd64.tar.gz
mipsel.deb	s390x.rpm	linux_arm64.tar.gz
ppc64el.deb	x86_64.rpm	linux_armel.tar.gz
riscv64.deb		linux_armhf.tar.gz
s390x.deb		linux_i386.tar.gz
		linux_mips.tar.gz
		linux_mipsel.tar.gz
		linux_ppc64le.tar.gz
		linux_riscv64.tar.gz
		linux_s390x.tar.gz
		static_linux_amd64.tar.gz

[MyaLongmire]

Thanks for fixing this so fast!

[Hipska]

Before this fix, my metrics were named "file" but now they are named "parser". (using xpath parser where I set a metric_name for each, this used to work before..