How to deal with NULL values on dynamic queries?

Hi,

I have a query that some values could be NULL, how can I deal with this problem?

[code]
  EXECUTE 'INSERT INTO '
        || quote_ident(pNOME_VIEW)
        || '('
        || quote_ident(pCHAVE_1)
        || ', DAT_INICIO, DAT_FIM, COMENT) values ('
        || quote_literal(pVAL_CHAVE_1)
        || ', '
        || quote_literal(pVAL_CHAVE_2)
        || ', '
        || quote_literal(pVAL_CAMPO1)
        || ', '
        || quote_literal(pVAL_COMENT)
        || ')';
[/code]

The variable pVAL_COMENT could be NULL or have a value. How can I deal with this?

Sorry the bad english.

Best Regards,