ESQL: Add LOOKUP JOIN tests with null and mv join keys (#118761) (#118779)

Notable behavior: - `null` join keys never match anything, even a `null`
from the left hand side. - If a lookup index document has multi-valued
join keys, it will match with all left hand side rows that contain any
of the multi-values. - If the left hand side has multi-values, it will
match with any lookup index document whose join key contains any of the
multi-values.

Author: alex-spies

x-pack/plugin/esql/qa/testFixtures/src/main/resources/languages_non_unique_key.csv

@@ -8,3 +8,7 @@ language_code:integer,language_name:keyword,country:keyword
 2,German,
 4,Quenya,
 5,,Atlantis
+[6,7],Mv-Lang,Mv-Land
+[7,8],Mv-Lang2,Mv-Land2
+,Null-Lang,Null-Land
+,Null-Lang2,Null-Land2

x-pack/plugin/esql/qa/testFixtures/src/main/resources/lookup-join.csv-spec

@@ -178,6 +178,73 @@ language_code:integer | language_name:keyword       | country:keyword
 2                     | [German, German, German]    | [Austria, Germany, Switzerland]
 ;
 
+nullJoinKeyOnTheDataNode
+required_capability: join_lookup_v5
+
+FROM employees
+| WHERE emp_no < 10004
+| EVAL language_code = emp_no % 10, language_code = CASE(language_code == 3, null, language_code)
+| LOOKUP JOIN languages_lookup_non_unique_key ON language_code
+| SORT emp_no
+| KEEP emp_no, language_code, language_name
+;
+
+emp_no:integer | language_code:integer | language_name:keyword
+10001          | 1                     | [English, English, English]
+10002          | 2                     | [German, German, German]
+10003          | null                  | null
+;
+
+
+mvJoinKeyOnTheDataNode
+required_capability: join_lookup_v5
+
+FROM employees
+| WHERE 10003 < emp_no AND emp_no < 10008
+| EVAL language_code = emp_no % 10
+| LOOKUP JOIN languages_lookup_non_unique_key ON language_code
+| SORT emp_no
+| KEEP emp_no, language_code, language_name
+;
+
+emp_no:integer | language_code:integer | language_name:keyword
+10004          | 4                     | Quenya
+10005          | 5                     | null
+10006          | 6                     | Mv-Lang
+10007          | 7                     | [Mv-Lang, Mv-Lang2]
+;
+
+mvJoinKeyFromRow
+required_capability: join_lookup_v5
+
+ROW language_code = [4, 5, 6, 7]
+| LOOKUP JOIN languages_lookup_non_unique_key ON language_code
+| EVAL language_name = MV_SORT(language_name), country = MV_SORT(country)
+| KEEP language_code, language_name, country
+;
+
+language_code:integer | language_name:keyword       | country:keyword
+[4, 5, 6, 7]          | [Mv-Lang, Mv-Lang2, Quenya] | [Atlantis, Mv-Land, Mv-Land2]
+;
+
+mvJoinKeyFromRowExpanded
+required_capability: join_lookup_v5
+
+ROW language_code = [4, 5, 6, 7, 8]
+| MV_EXPAND language_code
+| LOOKUP JOIN languages_lookup_non_unique_key ON language_code
+| EVAL language_name = MV_SORT(language_name), country = MV_SORT(country)
+| KEEP language_code, language_name, country
+;
+
+language_code:integer | language_name:keyword       | country:keyword
+4                     | Quenya                      | null
+5                     | null                        | Atlantis
+6                     | Mv-Lang                     | Mv-Land
+7                     | [Mv-Lang, Mv-Lang2]         | [Mv-Land, Mv-Land2]
+8                     | Mv-Lang2                    | Mv-Land2
+;
+
 lookupIPFromRow
 required_capability: join_lookup_v5