fix: Add existing_nullable usage to visit_column_type

Without this, altering operations (e.g. change column length) can cause nullability change of the column.
It then causes error if this column is a part of index:
"400 Changing NOT NULL constraints on column column_name is not allowed because it affects index index_name"

Author: minev-dev

google/cloud/sqlalchemy_spanner/sqlalchemy_spanner.py

@@ -1425,4 +1425,5 @@ def visit_column_type(
         alter_table(compiler, element.table_name, element.schema),
         alter_column(compiler, element.column_name),
         "%s" % format_type(compiler, element.type_),
+        "" if element.existing_nullable else "NOT NULL",
     )