fix: #2323 guard against null GPG_SECRING in uploadGpgKey (#2351)

Replace the implicit NullPointerException from
String.split with an explicit IOException naming
the missing environment variable, so a first-time
deployment without GPG_SECRING fails with an
actionable diagnostic instead of a stack trace
inside String.split.

Co-authored-by: edmoffo <9535571+edmoffo@users.noreply.github.com>

Author: edmoffo

src/main/java/com/rultor/agents/daemons/StartsDaemon.java

@@ -170,6 +170,11 @@ public String run(final XML xml) throws IOException {
      */
     private void uploadGpgKey(final Shell shell) throws IOException {
         final String secring = System.getenv("GPG_SECRING");
+        if (secring == null) {
+            throw new IOException(
+                "GPG_SECRING environment variable is not set"
+            );
+        }
         if (secring.split("\n").length < 10) {
             throw new IOException(
                 String.format(