If the `Mount` option inside a quadlet is missing the source=... part,
the code today panics with the following message.
```
panic: runtime error: index out of range [0] with length 0
goroutine 1 [running]:
github.com/containers/podman/v5/pkg/systemd/quadlet.handleStorageSource(0xc000140de0?, 0x1d?, {0x0?, 0x1?}, 0x5634e39e233e?, 0x10?)
...
```
This commit checks for the missing source and returns an error to avoid the panic.
Signed-off-by: Jakob Meier <mail@jakobmeier.ch>
