add nil check of showonly interface value

[BBBmau]

Description

fix a crash when setting show_only value to an empty string

Fixes #949

Acceptance tests

• Have you added an acceptance test for the functionality being added?

Release Note

Release note for CHANGELOG:

* Fix show_only crash in helm_template

References

Community Note

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[jrhouston]

Something you could also do here is use the type assertion to test this, as if it's nil then trying to assert it as a string will fail:

if s, ok := showFile.(string); ok {
  showFiles = append(showFiles, s)
}

[BBBmau]

This definitely is a much better way to do it. Thanks for the recommendation!

[alexsomesan]

LGTM. Hopefully this does the trick

[github-actions]

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