Bug: Substitution fails on prefix replacement with empty prefix

The following variable substitution is valid in bash:

```console
$ v=somevalue
$ echo "${v/#/prefix-}"
prefix-somevalue
```

When used with flux, this fails and the Kustomization controller throws an `envsubst error: unable to parse substitution within function` error. To reproduce, define a variable (any value), and reference it in a manner similar to `${SOME_VAR/#/prefix-}`.