[m365_defender] Bugfix for when entities fields are empty

[P1llus]

What does this PR do?

When entities fields are empty, it will be returned as a List rather than a Map or even Null, this makes the painless conditions break, as it is expecting a Map.

This also fixed a small condition error for mapping event.category properly in certain situations.

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
• I have verified that Kibana version constraints are current according to guidelines.

[elasticmachine]

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

[kcreddy]

LGTM 👍🏼

[ShourieG]

LGTM

[elasticmachine]

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• Start Time: 2022-12-19T09:49:01.166+0000

• Duration: 16 min 55 sec

Test stats 🧪

Test	Results
Failed	0
Passed	15
Skipped	0
Total	15

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[elasticmachine]

🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (2/2)	💚
Files	100.0% (2/2)	💚
Classes	100.0% (2/2)	💚
Methods	96.667% (29/30)	👎 -3.333
Lines	94.447% (2296/2431)	👎 -1.958
Conditionals	100.0% (0/0)	💚

[elasticmachine]

Package m365_defender - 1.4.2 containing this change is available at https://epr.elastic.co/search?package=m365_defender