Enable CLANG_WARN_NULLABLE_TO_NONNULL_CONVERSION for the projects.

objectivec/ProtocolBuffers_iOS.xcodeproj/project.pbxproj

diff --git a/objectivec/ProtocolBuffers_iOS.xcodeproj/project.pbxproj b/objectivec/ProtocolBuffers_iOS.xcodeproj/project.pbxproj
index 14e5103..e0a45ef 100644
--- a/objectivec/ProtocolBuffers_iOS.xcodeproj/project.pbxproj
+++ b/objectivec/ProtocolBuffers_iOS.xcodeproj/project.pbxproj
@@ -949,6 +949,7 @@
 				CLANG_ANALYZER_SECURITY_INSECUREAPI_STRCPY = YES;
 				CLANG_STATIC_ANALYZER_MODE = deep;
 				CLANG_WARN_DEPRECATED_OBJC_IMPLEMENTATIONS = YES;
+				CLANG_WARN_NULLABLE_TO_NONNULL_CONVERSION = YES;
 				CLANG_WARN_OBJC_IMPLICIT_ATOMIC_PROPERTIES = YES;
 				CLANG_WARN_OBJC_MISSING_PROPERTY_SYNTHESIS = YES;
 				CLANG_WARN_SUSPICIOUS_IMPLICIT_CONVERSION = YES;
@@ -994,6 +995,7 @@
 				CLANG_ANALYZER_SECURITY_INSECUREAPI_STRCPY = YES;
 				CLANG_STATIC_ANALYZER_MODE = deep;
 				CLANG_WARN_DEPRECATED_OBJC_IMPLEMENTATIONS = YES;
+				CLANG_WARN_NULLABLE_TO_NONNULL_CONVERSION = YES;
 				CLANG_WARN_OBJC_IMPLICIT_ATOMIC_PROPERTIES = YES;
 				CLANG_WARN_OBJC_MISSING_PROPERTY_SYNTHESIS = YES;
 				CLANG_WARN_SUSPICIOUS_IMPLICIT_CONVERSION = YES;