[BUG] Pushdown not work with expression `OR isnull(x)`

[LantaoJin]

What is the bug?
Pushdown not work with expression OR isnull(x)

POST /_plugins/_ppl/_explain
{
  "query": """
source=account | where firstname = 'Amber' or isnull(gender)
  """
}

returns

{
  "calcite": {
    "logical": """LogicalSystemLimit(fetch=[10000], type=[QUERY_SIZE_LIMIT])
  LogicalProject(account_number=[$0], firstname=[$1], address=[$2], balance=[$3], gender=[$4], city=[$5], employer=[$6], state=[$7], age=[$8], email=[$9], lastname=[$10])
    LogicalFilter(condition=[OR(=($1, 'Amber'), IS NULL($4))])
      CalciteLogicalIndexScan(table=[[OpenSearch, account]])
""",
    "physical": """EnumerableLimit(fetch=[10000])
  EnumerableCalc(expr#0..10=[{inputs}], expr#11=['Amber':VARCHAR], expr#12=[=($t1, $t11)], expr#13=[IS NULL($t4)], expr#14=[OR($t12, $t13)], proj#0..10=[{exprs}], $condition=[$t14])
    CalciteEnumerableIndexScan(table=[[OpenSearch, account]], PushDownContext=[[PROJECT->[account_number, firstname, address, balance, gender, city, employer, state, age, email, lastname]], OpenSearchRequestBuilder(sourceBuilder={"from":0,"timeout":"1m","_source":{"includes":["account_number","firstname","address","balance","gender","city","employer","state","age","email","lastname"],"excludes":[]}}, requestedTotalSize=2147483647, pageSize=null, startFrom=0)])
"""
  }
}

which the predicate pushdown does not work.