Fix Clang-3.8 compilation of tests

As far as I know, the compilation error is a compiler bug, but I
do not want to just drop the support for something that is trivial
to work around.

tests/SelfTest/IntrospectiveTests/String.tests.cpp

@@ -130,7 +130,7 @@ TEST_CASE("StringRef at compilation time", "[Strings][StringRef][constexpr]") {
     //  * substr
     using Catch::StringRef;
     SECTION("Simple constructors") {
-        constexpr StringRef empty;
+        constexpr StringRef empty{};
         STATIC_REQUIRE(empty.size() == 0);
         STATIC_REQUIRE(empty.begin() == empty.end());