cpanel_uapi: don't spill a redirection error when the key file is absent

With --signcsr the private key never exists in the cert home, so every
renewal printed ".../domain.key: No such file or directory" from the
shell redirection. Skip the key read in that case; the install_ssl call
already ran with an empty key there and cPanel keeps the installed one.

https://github.com/acmesh-official/acme.sh/issues/6228

deploy/cpanel_uapi.sh

@@ -52,7 +52,15 @@ cpanel_uapi_deploy() {
 
   # read cert and key files and urlencode both
   _cert=$(_url_encode <"$_ccert")
-  _key=$(_url_encode <"$_ckey")
+  # with --signcsr the private key was never handed to acme.sh, so the key
+  # file does not exist; skip it instead of spilling a shell redirection
+  # error on every renewal (cPanel keeps using the already-installed key)
+  if [ -f "$_ckey" ]; then
+    _key=$(_url_encode <"$_ckey")
+  else
+    _debug "Key file $_ckey does not exist (csr mode), not sending a key."
+    _key=""
+  fi
 
   _debug2 _cert "$_cert"
   _debug2 _key "$_key"