Fix for NPE when header values are null

[muralicmr]

Pull Request type

• Bugfix
• Feature
• Refactoring (no functional changes, no api changes)
• Build related changes (Please run ./gradlew generateLock saveLock to refresh dependencies)
• WHOSUSING.md
• Other (please describe):

Changes in this PR

Fix NPE when optional headers like below are not provided.

"headers": {
    "header1": "${workflow.input.header1}"
}

Describe the new behavior from this PR, and why it's needed
Issue #2884
In the new version of conductor HTTP tasks fail with NPE is a header value is not provided. As HTTP task is generic it is a good practice to support header values which are defined in workflow but not sent while making a call to execute the workflow.
Also its good to note that this feature was working in 2.x version of conductor.

Alternatives considered

No alternatives considered

[apanicker-nflx]

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