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/Common/BaseImporter.cpp

@@ -271,7 +271,7 @@ std::string BaseImporter::GetExtension(const std::string &file) {
 
     // no file extension at all
     if (pos == std::string::npos) {
-        return "";
+        return std::string();
     }
 
     // thanks to Andy Maloney for the hint