Fix is_positive_num() returning true for empty strings

To account for empty strings, we simply have to special-case them.
Simple as that.

This was also a problem with the previous std::string implementation of
this function; regardless, this is fixed now.

desktop_version/src/UtilityClass.cpp

@@ -286,6 +286,11 @@ static bool VVV_isxdigit(const unsigned char digit)
 
 bool is_positive_num(const char* str, const bool hex)
 {
+	if (str[0] == '\0')
+	{
+		return false;
+	}
+
 	for (size_t i = 0; str[i] != '\0'; ++i)
 	{
 		if (hex)