fix(parsers.xpath): Fix panic for JSON name expansion

[srebhan]

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

When using name-expansion for JSON documents containing mixes of arrays and objects, the xpath parser causes a panic

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x98 pc=0x438c0b7]

goroutine 88 [running]:
github.com/influxdata/telegraf/plugins/parsers/xpath.(*jsonDocument).GetNodePath(0xc0005d6a20?, {0x5c62b80?, 0xc0005dd020?}, {0x5c62b80?, 0xc0005dc7e0}, {0x6f29010, 0x1})
        /.../telegraf/plugins/parsers/xpath/json_document.go:47 +0x117
github.com/influxdata/telegraf/plugins/parsers/xpath.(*Parser).parseQuery(_, {_, _, _}, {_, _}, {_, _}, {{0xc000e62391, 0x5}, ...})
        /.../telegraf/plugins/parsers/xpath/parser.go:400 +0x1623
github.com/influxdata/telegraf/plugins/parsers/xpath.(*Parser).Parse(0xc0005d6a20, {0xc0009f6b00, 0x385, 0x580})
        /.../telegraf/plugins/parsers/xpath/parser.go:173 +0x58c
...

The panic is caused by non-array parent nodes of array elements. Those nodes do have a Value() of nil and thus reflect.TypeOf() panics. Fix the issue by checking for nil values and handle them correctly.

The test-case is taken from https://community.influxdata.com/t/json-v2-array-parsing/28529.

[telegraf-tiger]

Download PR build artifacts for linux_amd64.tar.gz, darwin_amd64.tar.gz, and windows_amd64.zip.
Downloads for additional architectures and packages are available below.

☺️ This pull request doesn't significantly change the Telegraf binary size (less than 1%)

📦 Click here to get additional PR build artifacts

Artifact URLs

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