Allow no-space after initial command

Addresses issue #2715

Marlin/Marlin_main.cpp

   // Args pointer optimizes code_seen, especially those taking XYZEF
   // This wastes a little cpu on commands that expect no arguments.
   current_command_args = current_command;
-  while (*current_command_args && *current_command_args != ' ') ++current_command_args;
+  while (*current_command_args >= '0' && *current_command_args <= '9') ++current_command_args;
   while (*current_command_args == ' ') ++current_command_args;
 
   // Interpret the code int