LibGUI: Prevent selecting empty line in TextEditor on double click (#6537)

Author: https://github.com/Sauler 🔰
Commit: c41c41cc0f
Pull-request: https://github.com/SerenityOS/serenity/pull/6537
Reviewed-by: https://github.com/IdanHo

Userland/Libraries/LibGUI/TextDocument.h

@@ -190,6 +190,7 @@ public:
     size_t first_non_whitespace_column() const;
     Optional<size_t> last_non_whitespace_column() const;
     bool ends_in_whitespace() const;
+    bool can_select() const;
     bool is_empty() const { return length() == 0; }
     size_t leading_spaces() const;