[SPARK-8844] [SPARKR] head/collect is broken in SparkR.

This is a WIP patch for SPARK-8844  for collecting reviews.

This bug is about reading an empty DataFrame. in readCol(),
      lapply(1:numRows, function(x) {
does not take into consideration the case where numRows = 0.

Will add unit test case.

Author: Sun Rui <rui.sun@intel.com>

Closes #7419 from sun-rui/SPARK-8844.