minor code improvements for the obj code

- make use of range for loops
- make use of empty() method for the std::vector
- removed unnecessary std::string initialization
- use ' ' instead of " " for the find methods (should be "faster")
- simplified some function
- make use of emplace_back instead of push_back (should be "faster")
- other small changes

code/Obj/ObjFileMtlImporter.cpp

@@ -274,7 +274,7 @@ void ObjFileMtlImporter::getFloatValue( ai_real &value )
 //  Creates a material from loaded data.
 void ObjFileMtlImporter::createMaterial()
 {
-    std::string line( "" );
+    std::string line;
     while( !IsLineEnd( *m_DataIt ) ) {
         line += *m_DataIt;
         ++m_DataIt;
@@ -282,7 +282,7 @@ void ObjFileMtlImporter::createMaterial()
 
     std::vector<std::string> token;
     const unsigned int numToken = tokenize<std::string>( line, token, " \t" );
-    std::string name( "" );
+    std::string name;
     if ( numToken == 1 ) {
         name = AI_DEFAULT_MATERIAL_NAME;
     } else {