Visual Studio Visual Assist code issues reported by VA code inspection of file json.hpp

[Andreas-Schniertshauer]

Code Issues reported by VA in json-3.9.1.zip in file json.hpp:
Line 3947 Unnecessary string initialisation: Member empty_str in template<typename IteratorType> class iteration_proxy_value
Line 4950 Unnecessary use of else after early exit: in class iterator_input_adapter in function get_character
Line 10621 Unnecessary use of else after early exit: in function bool sax_parse_internal(SAX* sax)
Line 14951 Unnecessary use of else after early exit: in function inline int find_largest_pow10(const std::uint32_t n, std::uint32_t& pow10)

[nlohmann]

The issue should be fixed with ffdeb77 in branch issue2615. @Andreas-Schniertshauer can you please check?