## What changes were proposed in this pull request?
This change initializes BUILD_ARGS to an empty array when $SPARK_HOME/RELEASE exists.
In function build, "local BUILD_ARGS" effectively creates an array of one element where the first and only element is an empty string, so "${BUILD_ARGS[]}" expands to "" and passes an extra argument to docker.
Setting BUILD_ARGS to an empty array makes "${BUILD_ARGS[]}" expand to nothing.
## How was this patch tested?
Manually tested.
$ cat RELEASE
Spark 2.3.0 (git revision a0d7949896) built for Hadoop 2.7.3
Build flags: -Phadoop-2.7 -Phive -Phive-thriftserver -Pkafka-0-8 -Pmesos -Pyarn -Pkubernetes -Pflume -Psparkr -DzincPort=3036
$ ./bin/docker-image-tool.sh -m t testing build
Sending build context to Docker daemon 256.4MB
...
vanzin
Author: Jooseong Kim <jooseong@pinterest.com>
Closes#20791 from jooseong/SPARK-23618.
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