Make FunctionRef support bool operator

Summary:
To support following code:
```
void foo(folly::FunctionRef<void(void)> callback = {}) {
  if (callback) {
    callback();
  }
}
```

Reviewed By: yfeldblum

Differential Revision: D4372296

fbshipit-source-id: 7d21e6a44b6f6b046b424f0139465511dbae7b8b

folly/Function.h

@@ -780,6 +780,10 @@ class FunctionRef<ReturnType(Args...)> final {
   ReturnType operator()(Args... args) const {
     return call_(object_, static_cast<Args&&>(args)...);
   }
+
+  explicit operator bool() const {
+    return object_;
+  }
 };
 
 } // namespace folly

folly/test/FunctionRefTest.cpp

@@ -137,11 +137,13 @@ TEST(FunctionRef, OverloadedFunctor) {
 TEST(FunctionRef, DefaultConstructAndAssign) {
   FunctionRef<int(int, int)> fref;
 
+  EXPECT_FALSE(fref);
   EXPECT_THROW(fref(1, 2), std::bad_function_call);
 
   int (*func)(int, int) = [](int x, int y) { return 10 * x + y; };
   fref = func;
 
+  EXPECT_TRUE(fref);
   EXPECT_EQ(42, fref(4, 2));
 }