If a File is Empty, Write Text to First Line

I need to check to see if two files are empty. If they are, then write "-999" to the file, else run an average on the numbers within the file.

Here is the coding I've been trying to use:

Code:

if [[ ! -z file1 ]] || [[ ! -z file2 ]]
then 

echo "-999" > file_avg

else

cat file1 >> tmp
cat file2 tmp > file_all
cat file_all | awk 'BEGIN {c = 0; sum = 0;} $1 ~ /^[0-9]*(\.[0-9]*)?$/ 
{a[c++] = $1; sum += $1;} END {avg = sum / c; if( (c % 2) == 1 ) {median = a[ int(c/2) ];}
else {median = ( a[c/2] + a[c/2-1] ) / 2;} OFS="\t"; print avg;}' file_all
> file_avg

fi

Contents of file_all:

76
76
78
79
75
76
77
78
76