Fix sqlparser crashes related to unknown tables

sqlparser/lib/src/analysis/schema/references.dart

@@ -44,7 +44,7 @@ class ReferenceScope {
     if (_availableColumns != null) return _availableColumns;
     if (inheritAvailableColumns) return parent.availableColumns;
 
-    return null;
+    return const [];
   }
 
   set availableColumns(List<Column> value) {

sqlparser/lib/src/analysis/steps/column_resolver.dart

@@ -63,25 +63,28 @@ class ColumnResolver extends RecursiveVisitor<void, void> {
     visitChildren(e, arg);
   }
 
+  void _addIfResolved(AstNode node, TableReference ref) {
+    final table = _resolveTableReference(ref);
+    if (table != null) {
+      node.scope.availableColumns = table.resolvedColumns;
+    }
+  }
+
   @override
   void visitUpdateStatement(UpdateStatement e, void arg) {
-    final table = _resolveTableReference(e.table);
+    _addIfResolved(e, e.table);
-    e.scope.availableColumns = table.resolvedColumns;
     visitChildren(e, arg);
   }
 
   @override
   void visitInsertStatement(InsertStatement e, void arg) {
-    final table = _resolveTableReference(e.table);
+    _addIfResolved(e, e.table);
-    visitChildren(e, arg);
-    e.scope.availableColumns = table.resolvedColumns;
     visitChildren(e, arg);
   }
 
   @override
   void visitDeleteStatement(DeleteStatement e, void arg) {
-    final table = _resolveTableReference(e.from);
+    _addIfResolved(e, e.from);
-    e.scope.availableColumns = table.resolvedColumns;
     visitChildren(e, arg);
   }
 

sqlparser/lib/utils/find_referenced_tables.dart

@@ -30,7 +30,7 @@ class ReferencedTablesVisitor extends RecursiveVisitor<void, void> {
   }
 
   NamedResultSet /*?*/ _toResultSetOrNull(ResolvesToResultSet resultSet) {
-    var resolved = resultSet.resultSet;
+    var resolved = resultSet?.resultSet;
 
     while (resolved != null && resolved is TableAlias) {
       resolved = (resolved as TableAlias).delegate;

sqlparser/test/utils/find_referenced_tables.dart

@@ -57,4 +57,11 @@ void main() {
     expect(
         findWrittenTables(ctx.root), {TableWrite(logins, UpdateKind.insert)});
   });
+
+  test('ignores unresolved references', () {
+    final ctx = engine.analyze('UPDATE xzy SET foo = bar');
+
+    expect(findWrittenTables(ctx.root), isEmpty);
+    expect(ctx.errors, hasLength(3)); // unknown table, two unknown references
+  });
 }