fix(inputs.gnmi): Do not provide empty prefix for subscription request

[srebhan]

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

resolves #12273

Do use a nil pointer instead of an empty path for prefix when creating a subscription request. This is necessary for (at least one) Huawei router(s) as they do not implement prefix support.

[jrventer]

@srebhan Tested this and looks like it is working correctly thanks.

[rasanentimo]

I also tested this with SONiC-OS-4.0.2-Enterprise Base on Dell S5248F and it's working as expected

[powersj]

@srebhan #12272 is merged, did you want to rebase this and then land?

[srebhan]

@powersj exactly. Rebased now and ready for your comments! ;-)

[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_arm64.zip
armel.deb	armv6hl.rpm	freebsd_amd64.tar.gz	windows_i386.zip
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