packetbeat/route: fix test failure on Windows 2022

[efd6]

Proposed commit message

Windows 2022 appears to have added new text to the output of netsh. The text
is interpreted as having three fields. This impact on the behaviour of the
field splitting which expects five.

- Originally this would cause a panic due to bounds checking. This was fixed
  in #39757 by erroring on an unexpectedly short line.
- The fieldsN helper expects lines to be n fields or longer, but does not
  terminate on lines with fewer fields.

So treat the first blank line found in the table as the end of the table and
fix the fieldsN helper.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Disruptive User Impact

Author's Checklist

• [ ]

How to test this PR locally

Related issues

• Closes Flaky Test: packetbeat/route TestDefault - panic: runtime error: index out of range [3] with length 3 #39698

Use cases

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[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @efd6? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[elasticmachine]

Pinging @elastic/sec-linux-platform (Team:Security-Linux Platform)