Disallow Optional::operator=(nullptr_t) unless T is a pointer

Summary: The old implementation of folly::none inadvertently allowed disengaging an optional by writing `op = nullptr`. Disallow that and require `op = folly::none`.

Reviewed By: yfeldblum

Differential Revision: D12884724

fbshipit-source-id: b17bcf00b245069d8ea2d9bc3703b0fdcaa85c07

Author: chadaustin

ReactCommon/jsiexecutor/jsireact/JSINativeModules.cpp

@@ -45,7 +45,7 @@ Value JSINativeModules::getModule(Runtime& rt, const PropNameID& name) {
 }
 
 void JSINativeModules::reset() {
-  m_genNativeModuleJS = nullptr;
+  m_genNativeModuleJS = folly::none;
   m_objects.clear();
 }