Fix falsy value for filterText and insertText

'filterText' and 'insertText' fields of 'CompletionItem' can only have
"" or undefined as falsy value.

https://github.com/microsoft/language-server-protocol/issues/1222

src/types.zig

@@ -230,8 +230,8 @@ pub const CompletionItem = struct {
     label: []const u8,
     kind: Kind,
     textEdit: ?TextEdit = null,
-    filterText: ?[]const u8 = null,
-    insertText: ?[]const u8 = null,
+    filterText: []const u8 = "",
+    insertText: []const u8 = "",
     insertTextFormat: ?InsertTextFormat = .PlainText,
     detail: ?[]const u8 = null,
     documentation: ?MarkupContent = null,