BLI: support initializing empty FunctionRef with nullptr

This may sometimes be desired because it is more explicitely
shows that the `FunctionRef` will be empty.

source/blender/blenlib/BLI_function_ref.hh

@@ -110,6 +110,10 @@ template<typename Ret, typename... Params> class FunctionRef<Ret(Params...)> {
  public:
   FunctionRef() = default;
 
+  FunctionRef(std::nullptr_t)
+  {
+  }
+
   /**
    * A `FunctionRef` itself is a callable as well. However, we don't want that this
    * constructor is called when `Callable` is a `FunctionRef`. If we would allow this, it

source/blender/blenlib/tests/BLI_function_ref_test.cc

@@ -124,4 +124,10 @@ TEST(function_ref, CallSafeVoid)
   EXPECT_EQ(value, 1);
 }
 
+TEST(function_ref, InitializeWithNull)
+{
+  FunctionRef<int(int, int)> f{nullptr};
+  EXPECT_FALSE(f);
+}
+
 }  // namespace blender::tests