[HIVEMALL-71] Handle null values and add a unit Tests to RescaleUDF.

[wangyum]

What changes were proposed in this pull request?

Handle null values and add a unit Tests to RescaleUDF.

What type of PR is it?

Improvement

What is the Jira issue?

https://issues.apache.org/jira/browse/HIVEMALL-71

How was this patch tested?

unit tests

[coveralls]

Coverage increased (+0.1%) to 35.914% when pulling 464e7db on wangyum:HIVEMALL-71 into 6b462ae on apache:master.

[myui]

Thanks, I will take this PR on.

[coveralls]

Coverage increased (+0.4%) to 36.222% when pulling 661571d on wangyum:HIVEMALL-71 into 6b462ae on apache:master.

[coveralls]

Coverage increased (+0.1%) to 35.923% when pulling 661571d on wangyum:HIVEMALL-71 into 6b462ae on apache:master.

[myui]

@wangyum Thanks. Merged with some modifications.

select rescale(v,min,max) from (
    select cast(1.2 as float) as v, 1.0 as min, 10.0 as max
    union all
    select cast(null as float) as v, 1.0 as min, 10.0 as max
    union all
    select cast(1.2 as double) as v, 1.0 as min, 10.0 as max
    union all
    select cast(null as double) as v, 1.0 as min, 10.0 as max
    union all
    select cast(1.2 as double) as v, cast(1.0 as double) as min, cast(10.0 as float) as max
) t;

0.022222228
NULL
0.022222228
NULL
0.022222228