Only check Windows iso SHA1 if a SHA1 is available

quickget

@@ -526,8 +526,11 @@ function get_windows() {
 
     echo "Downloading ${WINDOWS_NAME}..."
     web_get "${DOWNLOAD_URL}" "${VM_PATH}" "${FILE_NAME}"
-    echo "${DOWNLOAD_SHA1}  ${FILE_NAME}" > "${VM_PATH}/SHA1SUMS"
 
+    # Windows 10 doesn't include a SHA1 sum
+    # Only check the integrity is SHA1 is available.
+    if [ -n "${DOWNLOAD_SHA1}" ]; then
+      echo "${DOWNLOAD_SHA1}  ${FILE_NAME}" > "${VM_PATH}/SHA1SUMS"
       echo "Checking SHA1SUMS..."
       cd "${VM_PATH}"
       if ! sha1sum --check SHA1SUMS --ignore-missing --status; then
@@ -537,6 +540,7 @@ function get_windows() {
           echo "All good."
       fi
       cd ..
+    fi
 
     echo "Downloading virtio-win.iso..."
     web_get "https://fedorapeople.org/groups/virt/virtio-win/direct-downloads/stable-virtio/virtio-win.iso" "${VM_PATH}"