[modular] fix: intent filters should be optional.

Because they were marked as non-optional (in ModuleManifest fidl
struct), but optional in the .cmx manifest, the FIDL calls to story
shell (which involve passing the ModuleManifest) have been failing
because of a non-nullable vector which was null.

This fix makes fuchsia::modular::ModuleManifest.intent_filters optional.

TEST=manually saw that tests were not reporting story shell FIDL errors.
A comprehensive test CL after this unbreaks people.

Change-Id: Id68eabefbc944f9bfe448b6c28fe88d178a19206
diff --git a/public/fidl/fuchsia.modular/module/module_manifest.fidl b/public/fidl/fuchsia.modular/module/module_manifest.fidl
index 31c57e7..c74f241 100644
--- a/public/fidl/fuchsia.modular/module/module_manifest.fidl
+++ b/public/fidl/fuchsia.modular/module/module_manifest.fidl
@@ -16,7 +16,7 @@
     string? suggestion_headline;
 
     /// A list of intents that this module is able to handle.
-    vector<IntentFilter> intent_filters;
+    vector<IntentFilter>? intent_filters;
 
     /// Identifies the pattern with which to compose this module with others.
     string? composition_pattern;