[Filebeat][Fortinet] Fixing kv split for when assign-ip is not an IP and for date checking when eventtime is missing

[P1llus]

What does this PR do?

Since the KV processor does typechecking while performing the splitting operations, it will fail if the assignIP field is of type string, even though it is removed later in the pipeline when its not in the correct format. This change moves the KV split to a temp field to bypass the datatype checking, and performs a check against the IP field before renaming the fields to its correct placement.

Why is it important?

Fixes an issue with KV in cases where eventtime is missing and assignip is not in the correct datatype.

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.

[elasticmachine]

Pinging @elastic/security-external-integrations (Team:Security-External Integrations)

[elasticmachine]

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Build stats

• Build Cause: [Pull request #22361 updated]

• Start Time: 2020-11-02T19:48:04.740+0000

• Duration: 51 min 3 sec

Test stats 🧪

Test	Results
Failed	0
Passed	1948
Skipped	259
Total	2207

[leehinman]

LGTM

[elasticmachine]

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Test stats 🧪

Test	Results
Failed	0
Passed	1948
Skipped	259
Total	2207