Replace String comparisons with "", String() to is_empty()

Also:
- Adds two stress tests to test_string.h
- Changes to .empty() on std::strings

tests/core/math/test_math.cpp

@@ -312,7 +312,7 @@ public:
 					}
 				}
 
-				if (name != String()) {
+				if (!name.is_empty()) {
 					namespace_stack[at_level] = name;
 				}