style fix - initializing and assigning empty std::string properly

std::string s(""); s = ""; calls the copy constructor, which in turn calls strlen(), … assigning a default-constructed string generates fewer instructions and is therefore preferred.

With C++11 uniform initialization, you’d simply write s = { } instead.

code/PostProcessing/RemoveRedundantMaterials.h

@@ -81,7 +81,7 @@ public:
     /** @brief Set list of fixed (inmutable) materials
      *  @param fixed See #AI_CONFIG_PP_RRM_EXCLUDE_LIST
      */
-    void SetFixedMaterialsString(const std::string& fixed = "") {
+    void SetFixedMaterialsString(const std::string& fixed = std::string()) {
         mConfigFixedMaterials = fixed;
     }