Fixes in `rep` with NULL and empty vectors

...n/src/com/oracle/truffle/r/nodes/builtin/base/Repeat.java

...src/com/oracle/truffle/r/runtime/data/RComplexVector.java

.../src/com/oracle/truffle/r/runtime/data/RDoubleVector.java

...ime/src/com/oracle/truffle/r/runtime/data/RIntVector.java

...time/src/com/oracle/truffle/r/runtime/data/RListBase.java

...src/com/oracle/truffle/r/runtime/data/RLogicalVector.java

...ime/src/com/oracle/truffle/r/runtime/data/RRawVector.java

.../src/com/oracle/truffle/r/runtime/data/RStringVector.java

...est/src/com/oracle/truffle/r/test/ExpectedTestOutput.test

...c/com/oracle/truffle/r/test/builtins/TestBuiltin_rep.java