[geary/wip/728002-webkit2] Mark ClientWebView::get_string_result and call sites as nullable.

[Michael Gratton <mjog src gnome org>]

commit e00535e5092235a004d770aa01abb5a76619ba69
Author: Michael James Gratton <mike vee net>
Date:   Fri Dec 2 00:55:50 2016 +1100

    Mark ClientWebView::get_string_result and call sites as nullable.

 src/client/components/client-web-view.vala         |    2 +-
 .../conversation-viewer/conversation-web-view.vala |   12 ++++++++----
 2 files changed, 9 insertions(+), 5 deletions(-)
---
diff --git a/src/client/components/client-web-view.vala b/src/client/components/client-web-view.vala
index 153aef3..4d78075 100644
--- a/src/client/components/client-web-view.vala
+++ b/src/client/components/client-web-view.vala
@@ -104,7 +104,7 @@ public class ClientWebView : WebKit.WebView {
         // XXX unref result?
     }
 
-    protected static string get_string_result(WebKit.JavascriptResult result)
+    protected static string? get_string_result(WebKit.JavascriptResult result)
         throws JSError {
         JS.GlobalContext context = result.get_global_context();
         JS.Value js_str_value = result.get_value();
diff --git a/src/client/conversation-viewer/conversation-web-view.vala 
b/src/client/conversation-viewer/conversation-web-view.vala
index bc28fa4..91743f4 100644
--- a/src/client/conversation-viewer/conversation-web-view.vala
+++ b/src/client/conversation-viewer/conversation-web-view.vala
@@ -37,16 +37,20 @@ public class ConversationWebView : ClientWebView {
     /**
      * Returns the current selection, for prefill as find text.
      */
-    public async string get_selection_for_find() throws Error{
-        WebKit.JavascriptResult result = yield this.run_javascript("geary.getSelectionForFind();", null);
+    public async string? get_selection_for_find() throws Error{
+        WebKit.JavascriptResult result = yield this.run_javascript(
+            "geary.getSelectionForFind();", null
+        );
         return get_string_result(result);
     }
 
     /**
      * Returns the current selection, for quoting in a message.
      */
-    public async string get_selection_for_quoting() throws Error {
-        WebKit.JavascriptResult result = yield this.run_javascript("geary.getSelectionForQuoting();", null);
+    public async string? get_selection_for_quoting() throws Error {
+        WebKit.JavascriptResult result = yield this.run_javascript(
+            "geary.getSelectionForQuoting();", null
+        );
         return get_string_result(result);
     }