akamai: handle empty value placeholder

[efd6]

Proposed commit message

akamai: handle empty value placeholder

Apparently Akamai use "{p}" to indicate an absent value for this field, so
remove it if it is present before handling headers.

The test sample was provided by the issue reporter and modified to match
test IP constraints.

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.
• I have verified that any added dashboard complies with Kibana's Dashboard good practices

Author's Checklist

• [ ]

How to test this PR locally

Related issues

• Fixes [Akamai]: Ingest Pipeline doesn't expect field and value "responseHeader":"{p}" and causes pipeline errors.  #15878

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