LibGUI: Fix crash when text_in_range() was called on an empty document

Author: https://github.com/ngc6302h
Commit: 2b269b27e7
Pull-request: https://github.com/SerenityOS/serenity/pull/5772
Issue: https://github.com/SerenityOS/serenity/issues/5760
Issue: https://github.com/SerenityOS/serenity/issues/5768
Reviewed-by: https://github.com/awesomekling

Userland/Libraries/LibGUI/TextDocument.cpp

@@ -315,6 +315,8 @@ String TextDocument::text() const
 
 String TextDocument::text_in_range(const TextRange& a_range) const
 {
+    if (is_empty())
+        return String("");
     auto range = a_range.normalized();
 
     StringBuilder builder;