Caused by rB85421c4fab02: Geometry Nodes: Attribute search drop-down.

Above commit treated SOCK_RGBA and SOCK_STRING the same in
std_node_socket_draw.
That would turn a RGBA button into a text button (for attribute search),
leading to trouble.
Note these were just treated the same prior to above commit because both
were doing the layout split.

Now just give RGBA sockets their own case.