shill: wimax: Replace NULL with nullptr.

BUG=None
TEST=`USE=wimax FEATURES=test emerge-$BOARD shill`

Change-Id: I94d6014881cff1040774a11a7c0f49a0d5b3fb48
Reviewed-on: https://chromium-review.googlesource.com/220566
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
Commit-Queue: Ben Chan <benchan@chromium.org>
diff --git a/mock_wimax_provider.cc b/mock_wimax_provider.cc
index c5d0dd7..c6b9485 100644
--- a/mock_wimax_provider.cc
+++ b/mock_wimax_provider.cc
@@ -9,7 +9,7 @@
 namespace shill {
 
 MockWiMaxProvider::MockWiMaxProvider()
-    : WiMaxProvider(NULL, NULL, NULL, NULL) {}
+    : WiMaxProvider(nullptr, nullptr, nullptr, nullptr) {}
 
 MockWiMaxProvider::~MockWiMaxProvider() {}