Clean up null assumptions for Xcode and CocoaPods classes (#83442)

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

diff --git a/packages/flutter_tools/lib/src/ios/xcodeproj.dart b/packages/flutter_tools/lib/src/ios/xcodeproj.dart
index dbe3bd2..481fbd7 100644
--- a/packages/flutter_tools/lib/src/ios/xcodeproj.dart
+++ b/packages/flutter_tools/lib/src/ios/xcodeproj.dart
@@ -389,7 +389,7 @@
   }
   /// Returns unique scheme matching [buildInfo], or null, if there is no unique
   /// best match.
-  String? schemeFor(BuildInfo buildInfo) {
+  String? schemeFor(BuildInfo? buildInfo) {
     final String expectedScheme = expectedSchemeFor(buildInfo);
     if (schemes.contains(expectedScheme)) {
       return expectedScheme;