PMM-13629 fix pmm managed npe issue

[BupycHuk]

PMM-13629

Link to the Feature Build: SUBMODULES-0

If this PR adds or removes or alters one or more API endpoints, please review and add or update the relevant API documents as well:

• API Docs updated

If this PR is related to some other PRs in this or other repositories, please provide links to those PRs:

• Links to related pull requests (optional).

[codecov]

Codecov Report

Attention: Patch coverage is 74.50980% with 39 lines in your changes missing coverage. Please review.

Project coverage is 43.52%. Comparing base (58f70a2) to head (0a19d38).

Files with missing lines	Patch %	Lines
managed/models/agent_model.go	77.35%	12 Missing ⚠️
managed/services/agents/agents.go	53.84%	3 Missing and 3 partials ⚠️
managed/services/inventory/agents.go	58.33%	5 Missing ⚠️
managed/services/management/mysql.go	0.00%	3 Missing ⚠️
managed/services/management/postgresql.go	0.00%	3 Missing ⚠️
managed/services/management/agent.go	91.30%	2 Missing ⚠️
managed/services/management/azure_database.go	0.00%	2 Missing ⚠️
managed/services/management/mongodb.go	0.00%	2 Missing ⚠️
managed/models/agent_helpers.go	95.45%	1 Missing ⚠️
managed/services/agents/handler.go	0.00%	1 Missing ⚠️
... and 2 more

Additional details and impacted files

@@            Coverage Diff             @@
##               v3    #3404      +/-   ##
==========================================
- Coverage   43.53%   43.52%   -0.02%     
==========================================
  Files         366      366              
  Lines       44238    44244       +6     
==========================================
- Hits        19261    19258       -3     
- Misses      23289    23302      +13     
+ Partials     1688     1684       -4     

Flag	Coverage Δ
admin	11.47% <ø> (-0.05%)	⬇️
agent	51.83% <ø> (ø)
managed	45.28% <74.50%> (-0.02%)	⬇️
vmproxy	68.57% <ø> (ø)

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[percona-robot]

🚫 [golangci] _{reported by reviewdog 🐶}
missing cases in switch of type models.AgentType: models.PMMAgentType, models.NodeExporterType, models.ProxySQLExporterType, models.ExternalExporterType, models.VMAgentType (exhaustive)