Initialize instance to nullptr

Summary:
In tests, the instance is never initialized and it's hard to guard against null when the
instance isn't initialized to nullptr (instead being random data).

Reviewed By: ximyu

Differential Revision: D16984717

fbshipit-source-id: 414539e1117abaad16df9f9b7d3b8dbb328e38d0

xplat/Flipper/FlipperClient.cpp

@@ -29,7 +29,7 @@
 namespace facebook {
 namespace flipper {
 
-static FlipperClient* kInstance;
+static FlipperClient* kInstance{nullptr};
 
 using folly::dynamic;