Fixed #19299 -- Fixed Nullification of Foreign Keys To CharFields

Thanks tunixman for the report and Baptiste Mispelon and
Shai Berger for reviews.

tests/model_fields/models.py

@@ -72,12 +72,21 @@ class BooleanModel(models.Model):
     string = models.CharField(max_length=10, default='abc')
 
 
+class PrimaryKeyCharModel(models.Model):
+    string = models.CharField(max_length=10, primary_key=True)
+
+
 class FksToBooleans(models.Model):
     """Model wih FKs to models with {Null,}BooleanField's, #15040"""
     bf = models.ForeignKey(BooleanModel)
     nbf = models.ForeignKey(NullBooleanModel)
 
 
+class FkToChar(models.Model):
+    """Model with FK to a model with a CharField primary key, #19299"""
+    out = models.ForeignKey(PrimaryKeyCharModel)
+
+
 class RenamedField(models.Model):
     modelname = models.IntegerField(name="fieldname", choices=((1, 'One'),))