raise “empty symbol” errors during parsing

...es/src/com/oracle/truffle/r/nodes/function/RCallNode.java

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

...ser/src/com/oracle/truffle/r/parser/ParserGeneration.java

...acle.truffle.r.parser/src/com/oracle/truffle/r/parser/R.g

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

...test/src/com/oracle/truffle/r/test/parser/TestParser.java