Empty ReplacementTransformer source value causes panic

[drewwalters96]

Describe the bug
Operators can specify a string source value in the ReplacementTransformer rather than a path. For example:

replacements:
- source:
    value: "http://[email protected]"
  target:
    objref:
      kind: Deployment
      name: source-controller
    fieldrefs: ["spec.template.spec.containers[name=manager].env[name=http_proxy].value%REPLACEMENT_HTTP_PROXY%"]

When this source value is an empty string, airshipctl panics:

replacements:
- source:
    value: ""
  target:
    objref:
      kind: Deployment
      name: source-controller
    fieldrefs: ["spec.template.spec.containers[name=manager].env[name=http_proxy].value%REPLACEMENT_HTTP_PROXY%"]

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0xbc799e]

goroutine 1 [running]:
opendev.org/airship/airshipctl/pkg/document/plugin/replacement.getValue(0xc000b62800, 0x98, 0x100, 0xc0003046c0, 0xd4a3a0, 0xc0004ab980, 0xd462e0)
        opendev.org/airship/airshipctl@/pkg/document/plugin/replacement/transformer.go:89 +0x4e
opendev.org/airship/airshipctl/pkg/document/plugin/replacement.(*plugin).Filter(0xc0004ab980, 0xc000b62800, 0x98, 0x100, 0x0, 0x0, 0xd7f7b4, 0x32, 0x1556760)
        opendev.org/airship/airshipctl@/pkg/document/plugin/replacement/transformer.go:72 +0x9a
main.main.func1(0xc0000ee5a0, 0x0)
        opendev.org/airship/airshipctl@/krm-functions/replacement-transformer/main.go:25 +0x7d
sigs.k8s.io/kustomize/kyaml/fn/framework.execute(0xc0000ee5a0, 0xc000283b80, 0xc000224840, 0x15556f0, 0x0, 0x0, 0xc000297d08, 0xc000297d30)
        sigs.k8s.io/kustomize/[email protected]/fn/framework/framework.go:560 +0x128
sigs.k8s.io/kustomize/kyaml/fn/framework.Command.func1(0xc000224840, 0x15556f0, 0x0, 0x0, 0x0, 0x0)
        sigs.k8s.io/kustomize/[email protected]/fn/framework/framework.go:258 +0x8a
github.com/spf13/cobra.(*Command).execute(0xc000224840, 0xc0000901b0, 0x0, 0x0, 0xc000224840, 0xc0000901b0)
        github.com/spf13/[email protected]/command.go:842 +0x460
github.com/spf13/cobra.(*Command).ExecuteC(0xc000224840, 0xc000283b80, 0xc000224840, 0xc00004e750)
        github.com/spf13/[email protected]/command.go:950 +0x349
github.com/spf13/cobra.(*Command).Execute(...)
        github.com/spf13/[email protected]/command.go:887
main.main()
        opendev.org/airship/airshipctl@/krm-functions/replacement-transformer/main.go:28 +0xed
couldn't execute function: exit status 2

Steps To Reproduce
Use an empty string as a source in a ReplacementTransformer.

Expected behavior
Should be able to use an empty string as a replacement source without airshipctl panicking.

[gtsteffaniak]

assign to me

[gtsteffaniak]

PS : https://review.opendev.org/c/airship/airshipctl/+/805085