Validate empty observatory URI for screenshot (#71451)

packages/flutter_tools/lib/src/commands/screenshot.dart

@@ -77,6 +77,9 @@ class ScreenshotCommand extends FlutterCommand {
         if (observatoryUri == null) {
           throwToolExit('Observatory URI must be specified for screenshot type $screenshotType');
         }
+        if (observatoryUri.isEmpty || Uri.tryParse(observatoryUri) == null) {
+          throwToolExit('Observatory URI "$observatoryUri" is invalid');
+        }
     }
   }
 

packages/flutter_tools/test/general.shard/screenshot_command_test.dart

@@ -15,8 +15,18 @@ void main() {
     });
 
     testUsingContext('rasterizer and skia screenshots require observatory uri', () async {
-      expect(() => ScreenshotCommand.validateOptions('rasterizer', null, null), throwsToolExit());
-      expect(() => ScreenshotCommand.validateOptions('skia', null, null), throwsToolExit());
+      expect(
+          () => ScreenshotCommand.validateOptions('rasterizer', null, null),
+          throwsToolExit(
+              message:
+                  'Observatory URI must be specified for screenshot type rasterizer'));
+      expect(
+          () => ScreenshotCommand.validateOptions('skia', null, null),
+          throwsToolExit(
+              message:
+                  'Observatory URI must be specified for screenshot type skia'));
+      expect(() => ScreenshotCommand.validateOptions('skia', null, ''),
+          throwsToolExit(message: 'Observatory URI "" is invalid'));
     });
 
     testUsingContext('device screenshots require device', () async {