[BUG] Fix setting of service_account from PodTemplate

[pvditt]

Tracking issue

#3613

Why are the changes needed?

When manually setting service_account_name in pod template on a flytekit task, the service account gets overridden by default (empty string).

@task(
    pod_template=PodTemplate(
        pod_spec=V1PodSpec(
            service_account_name="test-service-account",
            containers=[],
        )
    )
)
def say_hello() -> str:
    return "Hello, World! Hello World !"

What changes were proposed in this pull request?

Don't overwrite a pod's service account if it's already set.

How was this patch tested?

Unit test added here
Manually tested

Setup process

Screenshots

Check all the applicable boxes

• I updated the documentation accordingly.
• All new and existing tests passed.
• All commits are signed-off.

Related PRs

Docs link

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (2105f09) 58.88% compared to head (688405e) 58.92%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4536      +/-   ##
==========================================
+ Coverage   58.88%   58.92%   +0.04%     
==========================================
  Files         620      620              
  Lines       52431    52432       +1     
==========================================
+ Hits        30874    30896      +22     
+ Misses      19090    19071      -19     
+ Partials     2467     2465       -2     

Flag	Coverage Δ
unittests	58.92% <100.00%> (+0.04%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[welcome]

Congrats on merging your first pull request! 🎉