NPE on Field Constraint Validation for Fields with no (null) Constraints

[baalmael]

It is not required for Fields to have constraints, so field.constraints can be null.
This leads to an NPE when calling field.checkConstraintViolations(...)

java.lang.NullPointerException: Cannot invoke "java.util.Map.containsKey(Object)" because "this.constraints" is null

	at io.frictionlessdata.tableschema.field.Field.checkConstraintViolations(Field.java:285)

I am happy to work on a fix