Null values are not validated and cast correctly #133

Field.checkConstraintViolations currently returns a pattern violation even when the field:

• is not required, and
• the actual value is null,
• but the schema defines a pattern constraint.

Also for json fields (such as array), null values cause an NPE even when the field is not required

io.frictionlessdata.tableschema.exception.InvalidCastException: java.lang.NullPointerException: Cannot invoke "String.startsWith(String)" because "string" is null

	at io.frictionlessdata.tableschema.field.Field.castValue(Field.java:674)

This behavior seems wrong to me and it is inkonsistent with the python implementation. I am happy to work on a fix and create a PR.