Small nit about $output in biblio_style_classic()

Another small issue showing up with PHP NOTICES turned on concerns the use of the $output variable on approximately line 53 of biblio_style_classic.inc file in the biblio_style_classic() function. There is a possibility that line 53

$output .= '';

is executed without having entered the if statement just above. Hence, the variable $output can be in an uninitialized state. Adding $output= ""; just above the if statement removes this condition and hence the NOTICE statement.

Cheers!