expr returning null, undefined, or NaN strategy

[DanielJDufour]

if an expr returns null or undefined or NaN, we should follow a defined strategy

expr_nullish_strategy
"skip" - just skip inserting
"no-data" - replace with out_no_data if available and proceed as normal
"none" - don't do anything, pass along value to everything else

[DanielJDufour]

added insert_null_strategy, which toggle whether to insert or skip null values

this doesn't address auto-mapping null to no-data values though