Check zero size of files correctly (#487)

from man test:
-s FILE
    FILE exists and has a size greater than zero

-z STRING
    the length of STRING is zero

include/functions

@@ -726,7 +726,7 @@
         if [ $# -eq 0 ]; then ExitFatal "Missing parameter when calling FileIsEmpty function"; fi
         EMPTY=0
         LogText "Test: checking if file $1 is empty"
-        if [ -z $1 ]; then
+        if [ ! -s "$1" ]; then
             LogText "Result: file $1 is empty"
             EMPTY=1
             return 0
@@ -1489,7 +1489,7 @@
         # Amazon EC2
         if [ "${SHORT}" = "" ]; then
             LogText "Test: checking specific files for Amazon"
-            if [ -f /etc/ec2_version -a ! -z /etc/ec2_version ]; then
+            if [ -f /etc/ec2_version -a -s /etc/ec2_version ]; then
                 SHORT="amazon-ec2"
             else
                 LogText "Result: system not hosted on Amazon"

include/tests_time

@@ -428,7 +428,7 @@
         FOUND=0
         FILE="/etc/ntp/step-tickers"
         if [ -f ${FILE} ]; then
-            if [ -z ${FILE} ]; then
+            if [ ! -s "${FILE}" ]; then
                 LogText "Result: ${FILE} is empty. The step-tickers contain no configured NTP servers"
                 Display --indent 2 --text "- Checking NTP step-tickers file" --result "EMPTY FILE" --color YELLOW
                 ReportSuggestion ${TEST_NO} "Use step-tickers file for quicker time synchronization"