[SPARK-20291][SQL] NaNvl(FloatType, NullType) should not be cast to NaNvl(DoubleType, DoubleType)

## What changes were proposed in this pull request?

`NaNvl(float value, null)` will be converted into `NaNvl(float value, Cast(null, DoubleType))` and finally `NaNvl(Cast(float value, DoubleType), Cast(null, DoubleType))`.

This will cause mismatching in the output type when the input type is float.

By adding extra rule in TypeCoercion can resolve this issue.

## How was this patch tested?

unite tests.

Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: DB Tsai <dbt@netflix.com>

Closes #17606 from dbtsai/fixNaNvl.