[Azure Blob Storage] Allowing empty content encoding headers for system tests

[P1llus]

What does this PR do?

Adding a failsafe for when content encoding headers might be nil, especially for system tests.

Why is it important?

Required for Agent integration package system tests

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.

Author's Checklist

Testing instructions

When creating system tests for elastic-package, we use the Azure Blob Storage emulator: https://hub.docker.com/_/microsoft-azure-storage-azurite

Unfortunately the emulator itself does not return a ContentEncoding header, while the Cloud API will always ensure at least a default octet-stream is added if value is nil, we need to add the possibility for the Encoding Header to not be present to be able to continue the system tests.

Steps to reproduce:

1. Start up the emulator: docker run -p 10000:10000 mcr.microsoft.com/azure-storage/azurite azurite-blob --blobHost 0.0.0.0
2. Download stream and run go build : https://github.com/elastic/stream
3. Create a local file for testing, for example test.json, add a JSON object on a single line, like { "testmessage" : "success" }
4. Populate the emulator with the testdata using stream: ./stream log --retry=30 --addr=localhost -p azureblobstorage --azure-blob-storage-port=10000 --azure-blob-storage-container=azure-container1 ./test.json
5. Configure the filebeat input to test against the emulator The password is a public key, not pasted by mistake:

filebeat.inputs:
- type: azure-blob-storage
  id: my-azureblobstorage-id
  enabled: true
  storage_url: http://localhost:10000/devstoreaccount1/
  account_name: devstoreaccount1
  auth.shared_credentials.account_key: Eby8vdM02xNOcqFlqUwJPLlmEtlCDXJ1OUzFT50uSRZ6IFsuFq2UVErCz4I6tq/K1SZFPTOtr/KBHBeksoGMGw==
  max_workers: 10
  poll: true
  poll_interval: 20s
  containers:
  - name: azure-container1
    max_workers: 1
    poll: true
    poll_interval: 20s

Before this PR, when we run filebeat with ./filebeat -e it will result in a panic:

panic: runtime error: invalid memory address or nil pointer dereference","component"
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x561c9cc3ac49]

However with this PR, the test will pass.

[elasticmachine]

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

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @P1llus? 🙏.
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

[ShourieG]

LGTM

[elasticmachine]

💚 Build Succeeded

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Build stats

• Start Time: 2023-01-30T08:53:54.714+0000

• Duration: 74 min 33 sec

Test stats 🧪

Test	Results
Failed	0
Passed	2547
Skipped	172
Total	2719

💚 Flaky test report

Tests succeeded.

🤖 GitHub comments

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[rdner]

Could you please add instructions to the PR description how this new behaviour was tested?