api: Avoid allocating empty array in LoadBalancer (grpc#12337)

Use a singleton empty Object array to initialize the customOptions
default in LoadBalancer.Builder.

Author: kilink

api/src/main/java/io/grpc/LoadBalancer.java

@@ -855,9 +855,11 @@ public String toString() {
     @ExperimentalApi("https://github.com/grpc/grpc-java/issues/1771")
     public static final class Builder {
 
+      private static final Object[][] EMPTY_CUSTOM_OPTIONS = new Object[0][2];
+
       private List<EquivalentAddressGroup> addrs;
       private Attributes attrs = Attributes.EMPTY;
-      private Object[][] customOptions = new Object[0][2];
+      private Object[][] customOptions = EMPTY_CUSTOM_OPTIONS;
 
       Builder() {
       }