fix(disk): keep disk info when df exits with errors

df still prints valid filesystems when a mount point cannot be queried (for example a stale FUSE mount), but exits non-zero. The dashboard and the disk alert discarded the whole output in that case, so no disk was displayed. Keep using the output whenever it is not empty. Refs #13864.

agent/app/service/alert.go

@@ -382,7 +382,7 @@ func executeDiskCommand() (string, error) {
 		cmdMgr2 := cmd.NewCommandMgr(cmd.WithTimeout(1 * time.Second))
 		stdout, err = cmdMgr2.RunWithStdout("df", "-lhT", "-P")
 	}
-	if err != nil {
+	if err != nil && strings.TrimSpace(stdout) == "" {
 		return stdout, err
 	}
 	var lines []string

agent/app/service/dashboard.go

@@ -469,7 +469,7 @@ func loadDiskInfo() []dto.DiskInfo {
 			cmd.PipeCommand{Name: "df", Args: []string{"-lhT", "-P"}},
 			cmd.PipeCommand{Name: "awk", Args: []string{format}},
 		)
-		if err != nil {
+		if err != nil && strings.TrimSpace(stdout) == "" {
 			return datas
 		}
 	}