'null' value support udf

I have a date of birth (dob) column in my source postgres table. I'm creating a realtime table in Pinot and using datatype:timestamp in my schema but this ends up defaulting the 'null' dob values to '1970-01-01' which is incorrect. So instead I'm using datatype:string and using the transformConfigs in table config to convert it.

Below is what I've tried out.

Error on the 'null' values when trying to use the 'todatetime' function.

`,
"ingestionConfig": {
"transformConfigs": [
{
"columnName": "dob",
"transformFunction": "toDateTime(date_of_birth,'yyyy-MM-dd')"
}
]
},`

Throws this error:

`java.lang.IllegalStateException: Caught exception while invoking method: public static java.lang.String org.apache.pinot.common.function.scalar.DateTimeFunctions.toDateTime(long,java.lang.String) with arguments: [null, yyyy-MM-dd]`

Attempted using Groovy but it does not work with Pinot builtin functions.

` "ingestionConfig": {
"transformConfigs": [
{
"columnName": "dob",
"transformFunction": "Groovy({date_of_birth != 'null' ? toDateTime(date_of_birth,'yyyy-MM-dd'): '1900-01-01'},date_of_birth)"
}
]
},`

Give this error:
`groovy.lang.MissingMethodException: No signature of method: Script1.toDateTime() is applicable for argument types: (java.lang.Long, java.lang.String) values: [304992000000, yyyy-MM-dd]`

Request is to add null support udf.

Pinot version: 0.8.0

Start with org.apache.pinot.common.function.scalar.DateTimeFunctions.toDateTime and the transformConfigs ingestion configuration shown in the issue. Reproduce the conversion with a null date_of_birth and compare it with the Groovy example; done means the requested null-support UDF behavior works for the realtime table without the reported exception.

Written by the indexing model from the issue text.