private long getSourceFileLength(List<Metadata> metadataListA, List<Metadata> metadataListB) {
        long len = getSourceFileLength(metadataListA);
        if (len > NON_EXISTENT_FILE_LENGTH) {
            return len;
        }
        return getSourceFileLength(metadataListB);
    }
Checks only the first item in each list. Returns the first
digest key shared by both, if it exists, null otherwise.

@param metadataListA
@param metadataListB
@return
tika-eval/tika-eval-app/src/main/java/org/apache/tika/eval/app/ExtractComparer.java