workaround for #12185 - stop using is_nothing

distribution/lib/Standard/Table/0.0.0-dev/src/Internal/Read_Many_Helpers.enso

@@ -77,26 +77,26 @@ _merge_input_and_tables (input_table : Table) (tables_for_rows : Vector Read_Man
 
         multiplicated_inputs = duplicate_rows input_table counts
         Runtime.assert (unified_data.row_count == multiplicated_inputs.row_count)
-        Runtime.assert (unified_metadata.is_nothing || (unified_metadata.row_count == unified_data.row_count))
+        Runtime.assert ((Nothing == unified_metadata) || (unified_metadata.row_count == unified_data.row_count))
 
-        first_pass = if unified_metadata.is_nothing then multiplicated_inputs else
+        first_pass = if Nothing == unified_metadata then multiplicated_inputs else
             multiplicated_inputs.zip unified_metadata right_prefix=""
         first_pass.zip unified_data right_prefix=""
 
 ## Unifies provided metadata tables, knowing that some tables may have no 
    metadata - in such case we want to insert as many Nothing rows for metadata 
    as there are rows in the corresponding data table.
 _unify_metadata (tables : Vector Read_Many_As_Table_Result) (on_problems : Problem_Behavior) -> Table | Nothing =
-    has_no_metadata = tables.all r-> r.metadata.is_nothing
+    has_no_metadata = tables.all r-> Nothing == r.metadata
     if has_no_metadata then Nothing else
         unique = Column_Naming_Helper.in_memory.create_unique_name_strategy
         tables.each r->
-            if r.metadata.is_nothing.not then unique.mark_used r.metadata.column_names
+            if Nothing != r.metadata then unique.mark_used r.metadata.column_names
 
         # A dummy column because we cannot create a table with 0 columns, it will be removed after union. We find an unique name for it to avoid conflicts.
         dummy_column_name = unique.make_unique "_Internal_Placeholder_Column_"
         tables_for_union = tables.map r->
-            if r.metadata.is_nothing.not then r.metadata else
+            if Nothing != r.metadata then r.metadata else
                 Table.new [Column.from_repeated_item dummy_column_name Nothing r.data.row_count]
 
         # Metadata are always merged by-name and columns that appear only in some tables are kept.