rpm: Fix requirements installation on SUSE

If requires are empty for source.

icinga-build-rpm-binary

@@ -41,9 +41,16 @@ echo "[ Installing build dependencies ]"
 case "$ICINGA_BUILD_OS" in
   opensuse*|sles)
     (
-      set -ex
-      # shellcheck disable=SC2046
-      run-retry-timeout sudo zypper --non-interactive install $(rpm -qpR "${source_rpm}")
+      if ! requires="$(rpm -qpR "${source_rpm}")"; then
+        echo "Could not lookup requires in source RPM!" >&2
+        exit 1
+      fi
+
+      echo "Found required: $requires"
+      if [ -n "$requires" ]; then
+        set -ex
+        run-retry-timeout sudo zypper --non-interactive install
+      fi
     )
     ;;
   *)