SUP-2875 | produce actionable error on missing default branch

[mcncl]

Changes

We can check the returned value of the pipeline targeted as it's a field on the struct

if p.DefaultBranch == "" {
	actionErr = fmt.Errorf("No default branch set for pipeline %s. Please specify a branch using --branch (-b)", pipeline)
	return
}

Here we just check that the default_branch of the pipeline isn't null, if it is then we produce a helpful error to use --branch in the command, else the build will be created on the default branch for the pipeline.`

[mcncl]

This is Go's way of checking that there is some value to a string, we don't care what that value is (foo is as valid as main for a default branch)