fix: don't dereference a nil release when the post-create lookup fails

[NickJosevski]

ReleaseService.GetByID returns (nil, err) on failure, but the warning branch after a successful release create read Assembled and ReleaseNotes off that nil result:

newlyCreatedRelease, lookupErr := octopus.Releases.GetByID(options.Response.ReleaseID)
if lookupErr != nil {
    cmd.PrintErrf("Warning: cannot fetch release details: %v\n", lookupErr)
    printReleaseVersion(options.Response.ReleaseVersion, newlyCreatedRelease.Assembled, ...)

So a transient server error in the window immediately after the release was created took the CLI down with a nil pointer dereference, instead of printing the warning that line was written to print. The release itself exists by that point, so the version still needs reporting.

Prints the version with empty details instead, and treats a nil release with no error as a lookup failure too.

Provenance

Pre-existing on main; found while reviewing #703 and originally fixed there. Lifting it into its own PR so it isn't gated on that stack, which isn't intended to merge as-is.

Verification

New release creation warns, rather than panicking, when the post-create lookup fails covers it: the post-create GET responds 500, and the command is expected to warn and still report the version. Confirmed it's a genuine regression test — reverting the create.go change makes it fail with panic: runtime error: invalid memory address or nil pointer dereference.

go build ./... clean, go test ./pkg/... green.

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