[Heartbeat] Fix tracking http url parsing error

[emilioalvap]

What does this PR do?

Fixes panics when http url is not parseable:

"[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x560c665b2eef]"
"goroutine 58 [running]:"
"github.com/elastic/beats/v7/heartbeat/monitors/wrappers.URLFields(0x0)"
"github.com/elastic/beats/v7/heartbeat/monitors/wrappers/util.go:56 +0x2f"

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.

How to test this PR locally

Configure an http monitor with urls: [" "]

[elasticmachine]

Pinging @elastic/uptime (Team:Uptime)

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @emilioalvap? 🙏.
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]

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• Start Time: 2023-03-01T09:55:50.201+0000

• Duration: 49 min 2 sec

Test stats 🧪

Test	Results
Failed	0
Passed	1921
Skipped	25
Total	1946

💚 Flaky test report

Tests succeeded.

🤖 GitHub comments

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[andrewvc]

LGTM

[vigneshshanmugam]

Good catch 👍🏽