cli: info: don't crash when channel has no schema (#584)

**Public-Facing Changes**
Fixed a bug where `mcap info` would crash if a channel had no schema.


**Description**
Fixes #579

go/cli/mcap/cmd/info.go

@@ -96,7 +96,13 @@ func printInfo(w io.Writer, info *mcap.Info) error {
 		if info.Statistics != nil {
 			row = append(row, fmt.Sprintf("%*d msgs (%.2f Hz)", maxCountWidth, channelMessageCount, frequency))
 		}
-		row = append(row, fmt.Sprintf(" : %s [%s]", schema.Name, schema.Encoding))
+		if schema != nil {
+			row = append(row, fmt.Sprintf(" : %s [%s]", schema.Name, schema.Encoding))
+		} else if channel.SchemaID != 0 {
+			row = append(row, fmt.Sprintf(" : <missing schema %d>", channel.SchemaID))
+		} else {
+			row = append(row, " : <no schema>")
+		}
 		rows = append(rows, row)
 	}
 	tw := tablewriter.NewWriter(buf)