Null check result of HttpServerBuilder supplier (#2317)

Null check result of `HttpServerBuilder` supplier
Motivation:
`DefaultGrpcServerBuilder` invokes a provided Supplier without checking
the result for null. A `NullPointerException` will occur, but will not
include context about what was null.
Modifications:
Add `Objects.requireNonNull` check when invoking `Supplier`
Result:
Better diagnostic for failed Supplier.

servicetalk-grpc-netty/src/main/java/io/servicetalk/grpc/netty/DefaultGrpcServerBuilder.java

@@ -93,8 +93,9 @@ final class DefaultGrpcServerBuilder implements GrpcServerBuilder, ServerBinder
 
     // Do not use this ctor directly, GrpcServers is the entry point for creating a new builder.
     DefaultGrpcServerBuilder(final Supplier<HttpServerBuilder> httpServerBuilderSupplier) {
-        this.httpServerBuilderSupplier = () -> httpServerBuilderSupplier.get()
-                .protocols(h2Default()).allowDropRequestTrailers(true);
+        this.httpServerBuilderSupplier = () ->
+                requireNonNull(httpServerBuilderSupplier.get(), "Supplier<HttpServerBuilder> result was null")
+                    .protocols(h2Default()).allowDropRequestTrailers(true);
     }
 
     @Override