See if an expression is blank
──────────────────────────────────────────────────────────────────────────────

 Syntax

        H_IsBlank( xExp )

 Arguments:

      xExp - An expression of any type

 Returns:

      .T. if the expression evaluates to a blank value
      .F. otherwise

 Description:

      The type of xExp is calculated, and zExp is then compared to a blank
      value of that type

 Examples:

      This should return .T. :

          H_IsBlank( CTOD( "  /  /  " ) )

 Source: ISBLANK.PRG

 Author:  Martin Colloby