Validate that removed fields are not required

Fixes jeremydaly#255
Defining a table attribute with `required: true` should mean that values always exist for that field (barring extraneous factors like schema changes and writes that may not go through this library). Updates may or may not specify a new value, but that new value should not be null, and the field cannot be removed outright.

This fixes two cases of this bug:
1) You can remove a `required: true` value by passing $remove: attr or `$remove: [attr]`
2) You can remove a `required: true` value by passing `attr: ''` if `removeNullAttributes` is set.

src/__tests__/entity.update.unit.test.ts

@@ -545,6 +545,22 @@ describe('update',()=>{
     })).toThrow(`'test2' is a required field`)
   })
 
+  it('fails when removing a required field', () => {
+    expect(() => TestEntity3.updateParams({
+      'pk': 'test-pk',
+      'test2': 'test-value',
+      '$remove': 'test'
+    })).toThrow(`'test' is required and cannot be removed`)
+  })
+
+  it('fails when setting a required field to a value that coerces to null', () => {
+    expect(() => TestEntity3.updateParams({
+      'pk': 'test-pk',
+      'test': '',
+      'test2': 'test-value',
+    })).toThrow(`'test' is required and cannot be removed`)
+  })
+
   it('fails when using non-numeric fields for indexed list updates', () => {
     expect(() => TestEntity.updateParams({
       'pk': 'test-pk',

src/classes/Entity.ts

@@ -784,12 +784,17 @@ class Entity<
           // Verify attribute is not a pk/sk
           if (schema.attributes[attrs[i]].partitionKey === true || schema.attributes[attrs[i]].sortKey === true)
             error(`'${attrs[i]}' is the ${schema.attributes[attrs[i]].partitionKey === true ? 'partitionKey' : 'sortKey' } and cannot be removed`)
+          // Verify attribute is not required
+          if (schema.attributes[attrs[i]].required)
+            error(`'${attrs[i]}' is required and cannot be removed`);
           // Grab the attribute name and add to REMOVE and names
           const attr = schema.attributes[attrs[i]].map || attrs[i]        
           REMOVE.push(`#${attr}`)
           names[`#${attr}`] = attr
         } // end for
       } else if (this._table!._removeNulls === true && (data[field] === null || String(data[field]).trim() === '') && (!mapping.link || mapping.save)) {
+        if (schema.attributes[field].required)
+          error(`'${field}' is required and cannot be removed`);
         REMOVE.push(`#${field}`)
         names[`#${field}`] = field
       } else if (