Fix parsing of macros that contain NN as well as other commands

[rmc47]

If a macro contains an NN command as well as others, since 26c29e1, that macro would be parsed as null by RDMacroEvent::load.

This PR ignores any null commands within a macro (is that the right behaviour or should they be preserved?), but tracks whether the macro contains only null commands. If so, it clears and returns false as before. Where the macro contains other commands as well as the NN, the other commands are loaded correctly, and only the NN is ignored.

To reproduce:

• Create a new macro cart in RDLibrary
• Add one regular line, like PN 1!
• Add a second line, NN ! (the default value if you double-click --- End of cart --- and click OK)
• Save the cart
• Edit the cart again

Expected behaviour: at least PN 1! is still visible; executing macro works.
Behaviour in v3.6.2: no lines (except --- End of cart---) are visible, and macro will not execute.