fix: command - always return non-nil command when parsing (fix #27)

To avoid panic when calling pcmd.Error in handlers.parse.

internal/command/set/sinter_test.go

@@ -37,6 +37,8 @@ func TestSInterParse(t *testing.T) {
 			testx.AssertEqual(t, err, test.err)
 			if err == nil {
 				testx.AssertEqual(t, cmd.keys, test.want.keys)
+			} else {
+				testx.AssertEqual(t, cmd, test.want)
 			}
 		})
 	}