made top level if checks gaurd clauses (#135070)

This is a tiny tweak to replace some top level if clauses with guard clauses in `FutureBuilder`. I find the resultant code much more readable, but this is a matter of taste and I didn't see any info one way or another on it in the style guide so let me know if this is not to your all's preference.

packages/flutter/lib/src/widgets/async.dart

@@ -595,14 +595,15 @@ class _FutureBuilderState<T> extends State<FutureBuilder<T>> {
   @override
   void didUpdateWidget(FutureBuilder<T> oldWidget) {
     super.didUpdateWidget(oldWidget);
-    if (oldWidget.future != widget.future) {
+    if (oldWidget.future == widget.future) {
+      return;
+    }
     if (_activeCallbackIdentity != null) {
       _unsubscribe();
       _snapshot = _snapshot.inState(ConnectionState.none);
     }
     _subscribe();
   }
-  }
 
   @override
   Widget build(BuildContext context) => widget.builder(context, _snapshot);
@@ -614,7 +615,10 @@ class _FutureBuilderState<T> extends State<FutureBuilder<T>> {
   }
 
   void _subscribe() {
-    if (widget.future != null) {
+    if (widget.future == null) {
+      // There is no future to subscribe to, do nothing.
+      return;
+    }
     final Object callbackIdentity = Object();
     _activeCallbackIdentity = callbackIdentity;
     widget.future!.then<void>((T data) {
@@ -642,7 +646,6 @@ class _FutureBuilderState<T> extends State<FutureBuilder<T>> {
       _snapshot = _snapshot.inState(ConnectionState.waiting);
     }
   }
-  }
 
   void _unsubscribe() {
     _activeCallbackIdentity = null;