how to check if $1 and $2 are null?

Try using the -z test:

if [ -z "$1" ] && [ -z "$2" ]

From man bash:

-z string
   True if the length of string is zero.

Since this is tagged bash, I recommend that one use the extended test construct ([[...]]), and forget the quotes:

if [[ -z $1 && -z $2 ]]; then
...

Unless you are going for sh/POSIX compatibility, there is no reason not to use [[ ]].

The following also works,

if [ "$1" == "" && "$2" == ""]; then
    echo NULL
fi

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