Bug: prefix substitution does not work with empty prefix string

[solidDoWant]

When using a prefix substitution with an empty string (e.g. ${SOME_VAR/#""/prefix-}), Flux fails to replace the prefix. No error is reported; flux simply does not replace the value. This is valid in bash as shown below:

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

[solidDoWant]

Please note that this has an subtle but important difference from #947. In this issue, an empty string (length 0) is explicitly provided, and in the other issue, no string is provided at all.