Default ExpConfigEntry.Id to "" in Java

The Id field is required by the wire contract, but defaulting to null let
class-level NON_NULL drop the key for a zero-value entry. Default it to ""
so the required key is always emitted, matching the Go and .NET defaults.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 43d7a02b-08ff-4e7d-a8c0-afa6dfbe94b0

Author: devm33

java/src/main/java/com/github/copilot/rpc/ExpConfigEntry.java

@@ -21,15 +21,15 @@
 public class ExpConfigEntry {
 
     @JsonProperty("Id")
-    private String id;
+    private String id = "";
 
     @JsonProperty("Parameters")
     private Map<String, Object> parameters = new LinkedHashMap<>();
 
     /**
      * Gets the identifier of this configuration entry.
      *
-     * @return the entry identifier, or {@code null} if not set
+     * @return the entry identifier (empty string when unset)
      */
     public String getId() {
         return id;