shell to sum 2 numbers to avoid error

To avoid a error when expression is incomplete in shell

error case:

# A=
# expr 1 + $A

expr: syntax error: missing argument after ‘+’

In this case 1 will be expected.

code example:

Both parameters should be checked if they are not empty.

#!/usr/bin/bash

function echo_sum() {

   if [[ -z $1 ]] && [[ -z $2 ]]; then echo 0;
   elif [[ -z $1 ]]; then echo `expr $2`;
   elif [[ -z $2 ]]; then echo `expr $1`;
   else echo `expr $1 + $2`;
   fi
 }
 A=300
 B=
 C=`echo_sum $A  $B`
 echo "C=$C"
 exit 0

result:

#  C=300