Fixed #17886 -- Fixed join promotion in ORed nullable queries

The ORM generated a query with INNER JOIN instead of LEFT OUTER JOIN
in a somewhat complicated case. The main issue was that there was a
chain of nullable FK -> non-nullble FK, and the join promotion logic
didn't see the need to promote the non-nullable FK even if the
previous nullable FK was already promoted to LOUTER JOIN. This resulted
in a query like a LOUTER b INNER c, which incorrectly prunes results.

tests/regressiontests/queries/models.py

@@ -385,3 +385,18 @@ class NullableName(models.Model):
 
     class Meta:
         ordering = ['id']
+
+class ModelD(models.Model):
+    name = models.TextField()
+
+class ModelC(models.Model):
+    name = models.TextField()
+
+class ModelB(models.Model):
+    name = models.TextField()
+    c = models.ForeignKey(ModelC)
+
+class ModelA(models.Model):
+    name = models.TextField()
+    b = models.ForeignKey(ModelB, null=True)
+    d = models.ForeignKey(ModelD)