[SPARK-13740][SQL] add null check for _verify_type in types.py

## What changes were proposed in this pull request?

This PR adds null check in `_verify_type` according to the nullability information.

## How was this patch tested?

new doc tests

Author: Wenchen Fan <wenchen@databricks.com>

Closes #11574 from cloud-fan/py-null-check.

python/pyspark/sql/types.py

@@ -1091,7 +1091,7 @@ _acceptable_types = {
 }
 
 
-def _verify_type(obj, dataType):
+def _verify_type(obj, dataType, nullable=True):
     """
     Verify the type of obj against dataType, raise a TypeError if they do not match.
 
@@ -1120,10 +1120,29 @@ def _verify_type(obj, dataType):
     Traceback (most recent call last):
         ...
     ValueError:...
+    >>> _verify_type(None, ByteType(), False) # doctest: +IGNORE_EXCEPTION_DETAIL
+    Traceback (most recent call last):
+        ...
+    ValueError:...
+    >>> _verify_type([1, None], ArrayType(ShortType(), False)) # doctest: +IGNORE_EXCEPTION_DETAIL
+    Traceback (most recent call last):
+        ...
+    ValueError:...
+    >>> _verify_type({None: 1}, MapType(StringType(), IntegerType()))
+    Traceback (most recent call last):
+        ...
+    ValueError:...
+    >>> schema = StructType().add("a", IntegerType()).add("b", StringType(), False)
+    >>> _verify_type((1, None), schema) # doctest: +IGNORE_EXCEPTION_DETAIL
+    Traceback (most recent call last):
+        ...
+    ValueError:...
     """
-    # all objects are nullable
     if obj is None:
-        return
+        if nullable:
+            return
+        else:
+            raise ValueError("This field is not nullable, but got None")
 
     # StringType can work with any types
     if isinstance(dataType, StringType):
@@ -1160,19 +1179,19 @@ def _verify_type(obj, dataType):
 
     elif isinstance(dataType, ArrayType):
         for i in obj:
-            _verify_type(i, dataType.elementType)
+            _verify_type(i, dataType.elementType, dataType.containsNull)
 
     elif isinstance(dataType, MapType):
         for k, v in obj.items():
-            _verify_type(k, dataType.keyType)
-            _verify_type(v, dataType.valueType)
+            _verify_type(k, dataType.keyType, False)
+            _verify_type(v, dataType.valueType, dataType.valueContainsNull)
 
     elif isinstance(dataType, StructType):
         if len(obj) != len(dataType.fields):
             raise ValueError("Length of object (%d) does not match with "
                              "length of fields (%d)" % (len(obj), len(dataType.fields)))
         for v, f in zip(obj, dataType.fields):
-            _verify_type(v, f.dataType)
+            _verify_type(v, f.dataType, f.nullable)
 
 
 # This is used to unpickle a Row from JVM