Replace operator null to exists

Author: MartinGotelli

examples/vectorstore.ipynb

@@ -235,8 +235,8 @@
     "\n",
     "The vectorstore supports a set of filters that can be applied against the metadata fields of the documents.\n",
     "\n",
-    "| Operator | Meaning/Category        |\n",
-    "|----------|-------------------------|\n",
+    "| Operator  | Meaning/Category        |\n",
+    "|-----------|-------------------------|\n",
     "| \\$eq      | Equality (==)           |\n",
     "| \\$ne      | Inequality (!=)         |\n",
     "| \\$lt      | Less than (<)           |\n",
@@ -246,7 +246,7 @@
     "| \\$in      | Special Cased (in)      |\n",
     "| \\$nin     | Special Cased (not in)  |\n",
     "| \\$between | Special Cased (between) |\n",
-    "| \\$null    | Special Cased (is null) |\n",
+    "| \\$exists  | Special Cased (is null) |\n",
     "| \\$like    | Text (like)             |\n",
     "| \\$ilike   | Text (case-insensitive like) |\n",
     "| \\$and     | Logical (and)           |\n",

tests/unit_tests/fixtures/filtering_test_cases.py

@@ -224,7 +224,7 @@
 ]
 
 TYPE_6_FILTERING_TEST_CASES = [
-    # These involve the special operator $null
+    # These involve the special operator $exists
     (
         {"happiness": {"$exists": True}},
         [],