RealmScreen: Assume navigation.state.params will exist.

There is no legitimate case in which we navigate to `RealmScreen`
without passing `params`, so it's not `RealmScreen`'s job to decide
what happens if `params` is missing.

Thankfully, we've centralized the navigate-to-`RealmScreen` logic in
`navigateToRealmScreen` in navActions.js, and, there, it's clear
that `params` is being passed.

In React Navigation v5, which is a dramatic departure from the
current API, we may get more robust type-checking, so that it's
feasible to catch any instances where we inadvertently don't pass
`params` [1].

[1] https://reactnavigation.org/docs/typescript/

src/start/RealmScreen.js

@@ -17,7 +17,7 @@ type SelectorProps = {|
 
 type Props = $ReadOnly<{|
   navigation: NavigationScreenProp<{
-    params: ?{|
+    params: {|
       realm: URL | void,
       initial?: boolean,
     |},
@@ -103,7 +103,7 @@ class RealmScreen extends PureComponent<Props, State> {
     return (
       <Screen
         title="Welcome"
-        canGoBack={!this.props.navigation.state.params?.initial}
+        canGoBack={!this.props.navigation.state.params.initial}
         padding
         centerContent
         keyboardShouldPersistTaps="always"
@@ -139,5 +139,5 @@ class RealmScreen extends PureComponent<Props, State> {
 }
 
 export default connect<SelectorProps, _, _>((state, props) => ({
-  initialRealm: props.navigation.state.params?.realm?.toString() ?? '',
+  initialRealm: props.navigation.state.params.realm?.toString() ?? '',
 }))(RealmScreen);