Added a null check for credentials during build time. This is a commo…

…n mistake when getting these values from env variables for example.

conductor-clients/java/conductor-java-sdk/orkes-client/src/main/java/io/orkes/conductor/client/ApiClient.java

@@ -150,6 +150,10 @@ public static ApiClientBuilder builder() {
     public static class ApiClientBuilder extends Builder<ApiClientBuilder> {
 
         public ApiClientBuilder credentials(String key, String secret) {
+            if (key == null || secret == null) {
+                throw new IllegalArgumentException("Key and secret must not be null");
+            }
+
             this.addHeaderSupplier(new OrkesAuthentication(key, secret));
             return this;
         }