fixed: failure to start when clipboard is empty

This patch checks the content of sdterr to make sure not to die when the
clipboard is read but empty.

cboard/tools.go

@@ -9,6 +9,8 @@ import (
 	"os/exec"
 )
 
+const ClipboardEmptyErrorString = "Nothing is copied\n"
+
 type toolsClipboardManager struct {
 }
 
@@ -33,9 +35,14 @@ func (c *toolsClipboardManager) Read() ([]byte, error) {
 
 	stdout := bytes.NewBuffer(nil)
 	cmd.Stdout = stdout
+	stderr := bytes.NewBuffer(nil)
+	cmd.Stderr = stderr
 
 	if err := cmd.Run(); err != nil {
-		return nil, fmt.Errorf("unable to run command: %w", err)
+		if stderr.String() == ClipboardEmptyErrorString {
+			return nil, nil
+		}
+		return nil, fmt.Errorf("unable to run read command: %w", err)
 	}
 
 	return stdout.Bytes(), nil
@@ -105,6 +112,10 @@ func (c *toolsClipboardManager) Watch(ctx context.Context) (<-chan []byte, <-cha
 					return
 				}
 
+				if len(data) <= 0 {
+					continue
+				}
+
 				select {
 				case chout <- data:
 				case <-ctx.Done():