Bug 1387002 - Replace .size() by .empty() when applicable in mfbt r=froydnj

MozReview-Commit-ID: 9S5oZQkykKv

--HG--
extra : rebase_source : dfc6b9bccb3a388486efb7179a5864b19e0952ec

mfbt/SmallPointerArray.h

@@ -181,7 +181,7 @@ private:
       return const_cast<T**>(&mInlineElements[0]);
     }
 
-    if (!mArray->size()) {
+    if (mArray->empty()) {
       return nullptr;
     }