Add antoher test case for coalesce & disable coalesce's doctest

Signed-off-by: Yuanchun Shen <yuanchu@amazon.com>

Author: yuancu

docs/user/ppl/functions/condition.rst

@@ -248,13 +248,13 @@ Version: 3.1.0
 
 Usage: coalesce(field1, field2, ...) return the first non-null value in the argument list.
 
-Argument type: all the supported data type, (NOTE : if parameters have different type, you will fail semantic check)
+Argument type: all the supported data type. The data types of all fields must be same.
 
 Return type: any
 
 Example::
 
-    os> source=accounts | eval result = coalesce(employer, firstname, lastname) | fields result, firstname, lastname, employer
+    PPL source=accounts | eval result = coalesce(employer, firstname, lastname) | fields result, firstname, lastname, employer
     fetched rows / total rows = 4/4
     +---------+-----------+----------+----------+
     | result  | firstname | lastname | employer |

integ-test/src/test/java/org/opensearch/sql/calcite/standalone/CalcitePPLConditionBuiltinFunctionIT.java

@@ -113,14 +113,20 @@ public void testCoalesce() {
     JSONObject actual =
         executeQuery(
             String.format(
-                "source=%s | where age = 10 | eval new_country = coalesce(name, state, country) | fields name, state, country, new_country",
+                "source=%s | where age = 10 | eval new_country = coalesce(name, state, country),"
+                    + " null = coalesce(name, state, name)  | fields name, state, country,"
+                    + " new_country, null",
                 TEST_INDEX_STATE_COUNTRY_WITH_NULL));
 
-    verifySchema(actual, schema("name", "string"), schema("state", "string"), schema("country", "string"), schema("new_country", "string"));
-
-    verifyDataRows(
+    verifySchema(
         actual,
-        rows(null, null, "Canada", "Canada"));
+        schema("name", "string"),
+        schema("state", "string"),
+        schema("country", "string"),
+        schema("new_country", "string"),
+        schema("null", "string"));
+
+    verifyDataRows(actual, rows(null, null, "Canada", "Canada", null));
   }
 
   @Test