envsubst -no-unset blows up on ${FOO:=}

It seems that an empty default value is treated as if a default value has not been provided.

Expected:

```console
~ $ go install github.com/a8m/envsubst/cmd/envsubst@v1.4.2
~ $ go/bin/envsubst -no-unset <<< 'value: ${FOO:=}'
value:
~ $ echo $?
0
~ $
```

Got:

```console
~ $ go install github.com/a8m/envsubst/cmd/envsubst@v1.4.2
~ $ go/bin/envsubst -no-unset <<< 'value: ${FOO:=}'
variable ${FOO} not set

~ $ echo $?
1
~ $
```