aliases: Treat empty pipe aliases as bad lines and skip them

A pipe alias must have a command, if it doesn't, we should treat the
line as bad and skip it like we do for others.

internal/aliases/aliases.go

@@ -307,6 +307,10 @@ func parseReader(domain string, r io.Reader) (map[string][]Recipient, error) {
 
 		if rawalias[0] == '|' {
 			cmd := strings.TrimSpace(rawalias[1:])
+			if cmd == "" {
+				// A pipe alias without a command is invalid.
+				continue
+			}
 			aliases[addr] = []Recipient{{cmd, PIPE}}
 		} else {
 			rs := []Recipient{}