Fix regression in hstring (#503)

Author: kennykerr

strings/base_string.h

@@ -257,12 +257,19 @@ WINRT_EXPORT namespace winrt
 
         const_pointer data() const noexcept
         {
-            return begin();
+            return c_str();
         }
 
         const_pointer c_str() const noexcept
         {
-            return begin();
+            if (!empty())
+            {
+                return begin();
+            }
+            else
+            {
+                return L"";
+            }
         }
 
         const_iterator begin() const noexcept

test/test/hstring_empty.cpp

@@ -0,0 +1,16 @@
+#include "pch.h"
+
+using namespace std::literals;
+
+TEST_CASE("hstring_empty")
+{
+    winrt::hstring s;
+    REQUIRE(s.empty());
+    REQUIRE(s.size() == 0);
+    REQUIRE(s == L""sv);
+    REQUIRE(std::distance(s.begin(), s.end()) == 0);
+
+    // Using wcslen to both validate that the strings are empty *and* that they are not null.
+    REQUIRE(wcslen(s.c_str()) == 0);
+    REQUIRE(wcslen(s.data()) == 0);
+}

test/test/test.vcxproj

@@ -363,6 +363,7 @@
     </ClCompile>
     <ClCompile Include="GetMany.cpp" />
     <ClCompile Include="get_activation_factory.cpp" />
+    <ClCompile Include="hstring_empty.cpp" />
     <ClCompile Include="iid_ppv_args.cpp" />
     <ClCompile Include="inspectable_interop.cpp">
       <PrecompiledHeader Condition="'$(Configuration)|$(Platform)'=='Debug|Win32'">NotUsing</PrecompiledHeader>