fix: correct omitempty tag in VariableFilter.EnvironmentScope field

Oleksandr Redkorequested to merge
alexandear/client-go-deletion_scheduled-70874095:fix/omitempty-struct-tag into main

Found this bug with the revive.struct-tag check:

$ golangci-lint run
project_variables.go:68:26: struct-tag: unknown option " omitempty" in url tag (revive)
1 issues:
* revive: 1

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