[HOTFIX] Copy SparkR lib if it exists in make-distribution

This is to fix an issue reported in #6373 where the `cp` would fail if `-Psparkr` was not used in the build

cc dragos pwendell

Author: Shivaram Venkataraman <shivaram@cs.berkeley.edu>

Closes #6379 from shivaram/make-distribution-hotfix and squashes the following commits:

08eb7e4 [Shivaram Venkataraman] Copy SparkR lib if it exists in make-distribution

make-distribution.sh

@@ -229,10 +229,13 @@ cp "$SPARK_HOME"/conf/*.template "$DISTDIR"/conf
 cp "$SPARK_HOME/README.md" "$DISTDIR"
 cp -r "$SPARK_HOME/bin" "$DISTDIR"
 cp -r "$SPARK_HOME/python" "$DISTDIR"
-mkdir -p "$DISTDIR"/R/lib
-cp -r "$SPARK_HOME/R/lib/SparkR" "$DISTDIR"/R/lib
 cp -r "$SPARK_HOME/sbin" "$DISTDIR"
 cp -r "$SPARK_HOME/ec2" "$DISTDIR"
+# Copy SparkR if it exists
+if [ -d "$SPARK_HOME"/R/lib/SparkR ]; then
+  mkdir -p "$DISTDIR"/R/lib
+  cp -r "$SPARK_HOME/R/lib/SparkR" "$DISTDIR"/R/lib
+fi
 
 # Download and copy in tachyon, if requested
 if [ "$SPARK_TACHYON" == "true" ]; then