Catch a panic when reading callExpressions without args

callExpressions can have any number of args. calling println() for
example would be a callExpr with no args.

println(someArray...) is another example where there is only one arg,
but it's expanded out into a variable number at runtime.
More idiomatic way of checking for a key in a map

Empty string is a valid value for a map. The index operation on maps can
return two values -- the second, bool value is helpful for checking
existence in a map.