simplify some argument cast nodes, remove special RNull / null mapping

...m/oracle/truffle/r/nodes/casts/PredefMappersSamplers.java

...acle/truffle/r/nodes/unary/ConditionalMapNodeSampler.java

...c/com/oracle/truffle/r/nodes/unary/FilterNodeSampler.java

.../src/com/oracle/truffle/r/nodes/unary/MapNodeSampler.java

...ffle/r/nodes/builtin/AbstractPredicateArgumentFilter.java

...s/src/com/oracle/truffle/r/nodes/builtin/CastBuilder.java

.../com/oracle/truffle/r/nodes/unary/ConditionalMapNode.java

...odes/src/com/oracle/truffle/r/nodes/unary/FilterNode.java

...r.nodes/src/com/oracle/truffle/r/nodes/unary/MapNode.java