${var:+FOO} mishandles empty vars

```console
$ export empty=''
$ echo '[${empty:+FOO}]' | envsubst
[FOO]
```
This is not what shell does:
```console
$ echo "[${empty:+FOO}]"
[]
```

Look at the parsing and substitution logic in the Go source code, likely in a file like `envsubst.go` or `parser.go`. The issue demonstrates that the `${var:+FOO}` syntax incorrectly substitutes when the variable is set but empty. Compare the shell's behavior (Bash) to the current implementation. Write a test to reproduce the bug, then modify the substitution logic to handle empty strings correctly. Check for similar patterns like `${var:-FOO}` to ensure consistency.

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