chore: fix CreateBlueprintDisplayMetadata function when there is no existing input

[tjy9206]

I encountered this error when trying to generate metadata.display.yaml (running cft with -d flag):

INFO[10-18|17:39:03] Sort variables by the original input order. 
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x28 pc=0x1454fc5]

goroutine 1 [running]:
github.com/GoogleCloudPlatform/cloud-foundation-toolkit/cli/bpmetadata.CreateBlueprintDisplayMetadata({0xc000062c30?, 0x46?}, 0xc000ec4e40, 0xc000680cc0)
        /usr/local/google/home/jieyutian/cloud-foundation-toolkit/cli/bpmetadata/cmd.go:311 +0x365
github.com/GoogleCloudPlatform/cloud-foundation-toolkit/cli/bpmetadata.generateMetadataForBpPath({0xc000062c30, 0x46})
        /usr/local/google/home/jieyutian/cloud-foundation-toolkit/cli/bpmetadata/cmd.go:179 +0x205
github.com/GoogleCloudPlatform/cloud-foundation-toolkit/cli/bpmetadata.generate(0x49ffa00?, {0xc000680c60?, 0x4?, 0x2b7fbf1?})
        /usr/local/google/home/jieyutian/cloud-foundation-toolkit/cli/bpmetadata/cmd.go:125 +0x607
github.com/spf13/cobra.(*Command).execute(0x49ffa00, {0xc000680c00, 0x6, 0x6})
        /usr/local/google/home/jieyutian/go/pkg/mod/github.com/spf13/[email protected]/command.go:985 +0xaaa
github.com/spf13/cobra.(*Command).ExecuteC(0x49fe8c0)
        /usr/local/google/home/jieyutian/go/pkg/mod/github.com/spf13/[email protected]/command.go:1117 +0x3ff
github.com/spf13/cobra.(*Command).Execute(...)
        /usr/local/google/home/jieyutian/go/pkg/mod/github.com/spf13/[email protected]/command.go:1041
github.com/GoogleCloudPlatform/cloud-foundation-toolkit/cli/cmd.Execute()
        /usr/local/google/home/jieyutian/cloud-foundation-toolkit/cli/cmd/root.go:86 +0x1a
main.main()
        /usr/local/google/home/jieyutian/cloud-foundation-toolkit/cli/main.go:6 +0xf

This is because we are not checking if bpCore.Spec.Ui.Input has any null pointer references in existingInput := proto.Clone(bpCore.Spec.Ui.Input).(*BlueprintUIInput). Fixing in this PR.

[qz267]

LGTM, thanks @tjy9206