ledger-signer: Don't try to split cfgStr if none is specified

This should give a better error message when the user didn't specify a
valid signer configuration, i.e. it will return "address not configured"
instead of "malformed k/v pair: ''".

ledger-signer/ledger_signer.go

@@ -49,7 +49,12 @@ type pluginConfig struct {
 }
 
 func newPluginConfig(cfgStr string) (*pluginConfig, error) {
-	kvStrs := strings.Split(cfgStr, ";")
+	var kvStrs []string
+
+	// Don't try to split cfgStr if no configuration is specified.
+	if cfgStr != "" {
+		kvStrs = strings.Split(cfgStr, ";")
+	}
 
 	var (
 		cfg                      pluginConfig