Find all zero-byte files in directory and subdirectories

Bash 4+ tested - This is the correct way to search for size 0:

find /path/to/dir -size 0 -type f -name "*.xml"

Search for multiple file extensions of size 0:

find /path/to/dir -size 0 -type f \( -iname \*.css -o -iname \*.js \)

Note: If you removed the \( ... \) the results would be all of the files that meet this requirement hence ignoring the size 0.

As addition to the answers above:

If you would like to delete those files

find $dir -size 0 -type f -delete

No, you don't have to bother grep.

find $dir -size 0 ! -name "*.xml"

To print the names of all files in and below $dir of size 0:

find "$dir" -size 0

Note that not all implementations of find will produce output by default, so you may need to do:

find "$dir" -size 0 -print

Two comments on the final loop in the question:

Rather than iterating over every other word in a string and seeing if the alternate values are zero, you can partially eliminate the issue you're having with whitespace by iterating over lines. eg:

printf '1 f1\n0 f 2\n10 f3\n' | while read size path; do
    test "$size" -eq 0 && echo "$path"; done

Note that this will fail in your case if any of the paths output by ls contain newlines, and this reinforces 2 points: don't parse ls, and have a sane naming policy that doesn't allow whitespace in paths.

Secondly, to output the data from the loop, there is no need to store the output in a variable just to echo it. If you simply let the loop write its output to stdout, you accomplish the same thing but avoid storing it.