Stop certbot-auto from printing blank lines (#7016)

Fixes #7012.

Apparently, the previous test we had here doesn't catch the case when certbot-auto prints blank lines. (I don't yet understand why so if someone does, please let me know!)

Regardless, I fixed up the test and verified it fails with the version of letsencrypt-auto in master and then fixed letsencrypt-auto so the test passes.

I ran test farm tests on the changes here and they passed on all instances.

* correct test

* fixes #7012

letsencrypt-auto-source/letsencrypt-auto

@@ -1593,12 +1593,14 @@ UNLIKELY_EOF
     # ---------------------------------------------------------------------------
     # If the script fails for some reason, don't break certbot-auto.
     set +e
-    # Suppress unexpected error output and only print the script's output if it
-    # ran successfully.
+    # Suppress unexpected error output.
     CHECK_PERM_OUT=$("$LE_PYTHON" "$TEMP_DIR/check_permissions.py" "$0" 2>/dev/null)
     CHECK_PERM_STATUS="$?"
     set -e
-    if [ "$CHECK_PERM_STATUS" = 0 ]; then
+    # Only print output if the script ran successfully and it actually produced
+    # output. The latter check resolves
+    # https://github.com/certbot/certbot/issues/7012.
+    if [ "$CHECK_PERM_STATUS" = 0 -a -n "$CHECK_PERM_OUT" ]; then
       error "$CHECK_PERM_OUT"
     fi
   fi

letsencrypt-auto-source/letsencrypt-auto.template

@@ -670,12 +670,14 @@ UNLIKELY_EOF
     # ---------------------------------------------------------------------------
     # If the script fails for some reason, don't break certbot-auto.
     set +e
-    # Suppress unexpected error output and only print the script's output if it
-    # ran successfully.
+    # Suppress unexpected error output.
     CHECK_PERM_OUT=$("$LE_PYTHON" "$TEMP_DIR/check_permissions.py" "$0" 2>/dev/null)
     CHECK_PERM_STATUS="$?"
     set -e
-    if [ "$CHECK_PERM_STATUS" = 0 ]; then
+    # Only print output if the script ran successfully and it actually produced
+    # output. The latter check resolves
+    # https://github.com/certbot/certbot/issues/7012.
+    if [ "$CHECK_PERM_STATUS" = 0 -a -n "$CHECK_PERM_OUT" ]; then
       error "$CHECK_PERM_OUT"
     fi
   fi

tests/letstest/scripts/test_letsencrypt_auto_certonly_standalone.sh

@@ -42,8 +42,8 @@ if ! letsencrypt-auto --help --no-self-upgrade | grep -F "letsencrypt-auto [SUBC
     exit 1
 fi
 
-OUTPUT=$(letsencrypt-auto --install-only --no-self-upgrade --quiet 2>&1)
-if [ -n "$OUTPUT" ]; then
+OUTPUT_LEN=$(letsencrypt-auto --install-only --no-self-upgrade --quiet 2>&1 | wc -c)
+if [ "$OUTPUT_LEN" != 0 ]; then
     echo letsencrypt-auto produced unexpected output!
     exit 1
 fi