Specify nullability in ProtoRPC

src/objective-c/ProtoRPC/ProtoRPC.h

@@ -134,10 +134,11 @@ __attribute__((deprecated("Please use GRPCProtoCall."))) @interface ProtoRPC
        * addr and the port number, for example @"localhost:5050".
        */
       -
-      (instancetype)initWithHost : (NSString *)host method
+      (instancetype _Null_unspecified)initWithHost : (NSString *_Null_unspecified)host method
-    : (GRPCProtoMethod *)method requestsWriter : (GRXWriter *)requestsWriter responseClass
+    : (GRPCProtoMethod *_Null_unspecified)method requestsWriter
-    : (Class)responseClass responsesWriteable
+    : (GRXWriter *_Null_unspecified)requestsWriter responseClass
-    : (id<GRXWriteable>)responsesWriteable NS_DESIGNATED_INITIALIZER;
+    : (Class _Null_unspecified)responseClass responsesWriteable
+    : (id<GRXWriteable> _Null_unspecified)responsesWriteable NS_DESIGNATED_INITIALIZER;
 
 - (void)start;
 @end