Fix potential NPE in iOS doctor check (#8016)

* Fix potential NPE in iOS doctor check

In case Xcode is not installed, the xcode-select path may be null.

* fixup! Fix potential NPE in iOS doctor check

packages/flutter_tools/lib/src/ios/ios_workflow.dart

@@ -87,7 +87,7 @@ class IOSWorkflow extends DoctorValidator implements Workflow {
       }
     } else {
       xcodeStatus = ValidationType.missing;
-      if (xcode.xcodeSelectPath.isEmpty) {
+      if (xcode.xcodeSelectPath == null || xcode.xcodeSelectPath.isEmpty) {
         messages.add(new ValidationMessage.error(
             'Xcode not installed; this is necessary for iOS development.\n'
             'Download at https://developer.apple.com/xcode/download/.'

packages/flutter_tools/lib/src/ios/mac.dart

@@ -29,8 +29,8 @@ class Xcode {
     _eulaSigned = false;
 
     try {
-      _xcodeSelectPath = runSync(<String>['xcode-select', '--print-path']);
-      if (_xcodeSelectPath == null || _xcodeSelectPath.trim().isEmpty) {
+      _xcodeSelectPath = runSync(<String>['xcode-select', '--print-path'])?.trim();
+      if (_xcodeSelectPath == null || _xcodeSelectPath.isEmpty) {
         _isInstalled = false;
         return;
       }

packages/flutter_tools/test/src/ios/ios_workflow_test.dart

@@ -20,7 +20,7 @@ void main() {
 
     testUsingContext('Emit missing status when nothing is installed', () async {
       when(xcode.isInstalled).thenReturn(false);
-      when(xcode.xcodeSelectPath).thenReturn('');
+      when(xcode.xcodeSelectPath).thenReturn(null);
       IOSWorkflowTestTarget workflow = new IOSWorkflowTestTarget()
         ..hasPythonSixModule = false
         ..hasHomebrew = false
@@ -31,7 +31,7 @@ void main() {
 
     testUsingContext('Emits partial status when Xcode is not installed', () async {
       when(xcode.isInstalled).thenReturn(false);
-      when(xcode.xcodeSelectPath).thenReturn('');
+      when(xcode.xcodeSelectPath).thenReturn(null);
       IOSWorkflowTestTarget workflow = new IOSWorkflowTestTarget();
       ValidationResult result = await workflow.validate();
       expect(result.type, ValidationType.partial);