Fix passing nullptr to args marked as non-null

libstdc++'s |memcpy| and |memset| both state that their inputs should
never be a nullptr since this matches the C spec. Some compilers act on
these hints, so we shouldn't unconditionally call these as it would
signal to the compiler that they can't be nullptrs.

As an example, the following code will always call |do_something()|
when compiled with optimisations since GCC version 4.9:
```
    void clear(void *ptr, int size) {
        memset(ptr, 0, size);
    }
    void example(void *ptr, int size) {
        clear(ptr, size);
        if (ptr != nullptr) do_something();
    }
```

src/string.cpp

@@ -156,6 +156,7 @@ gb_internal isize string_index_byte(String const &s, u8 x) {
 
 gb_internal gb_inline bool str_eq(String const &a, String const &b) {
 	if (a.len != b.len) return false;
+	if (a.len == 0) return true;
 	return memcmp(a.text, b.text, a.len) == 0;
 }
 gb_internal gb_inline bool str_ne(String const &a, String const &b) { return !str_eq(a, b);                }