## What changes were proposed in this pull request?

Added a check for for the number of defined values.  Previously the argmax function assumed that at least one value was defined if the vector size was greater than zero.

## How was this patch tested?

Tests were added to the existing VectorsSuite to cover this case.

Author: Jon McLean <jon.mclean@atsid.com>

Closes #17877 from jonmclean/vectorArgmaxIndexBug.

(cherry picked from commit be53a783)
Signed-off-by: Sean Owen <sowen@cloudera.com>