Remove implicit cast from CFX_ByteString to (const char*).

BUG=

Review URL: https://codereview.chromium.org/1885973002
diff --git a/fpdfsdk/javascript/global.cpp b/fpdfsdk/javascript/global.cpp
index d843b60..7242a14 100644
--- a/fpdfsdk/javascript/global.cpp
+++ b/fpdfsdk/javascript/global.cpp
@@ -414,7 +414,7 @@
                                               const CFX_ByteString& sData,
                                               v8::Local<v8::Object> pData,
                                               bool bDefaultPersistent) {
-  if (!propname)
+  if (propname.IsEmpty())
     return FALSE;
 
   auto it = m_mapGlobal.find(propname);