fix: prevent IndexError in adaptive relocation with auto_save

When `css()`/`xpath()` are called with both `adaptive=True` and
`auto_save=True`, the relocation branch guarded the re-save with
`if elements is not None`. However `relocate()` returns an empty
list (never `None`) when no candidate clears the `percentage`
threshold, so the guard always passed and `self.save(elements[0], ...)`
raised `IndexError: list index out of range`.

This crashes exactly when adaptive resilience is needed most: the page
structure changed enough that nothing matches above the threshold.

Fix: use a truthiness check (`if elements and auto_save`) so the
re-save is skipped when relocation yields nothing. The successful
relocation path (which re-saves the relocated element) is unchanged.

Added a regression test that fails before the fix (IndexError) and
passes after.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

scrapling/parser.py

@@ -671,7 +671,7 @@ class Selector(SelectorsGeneration):
                     element_data = self.retrieve(identifier or selector)
                     if element_data:
                         elements = self.relocate(element_data, percentage)
-                        if elements is not None and auto_save:
+                        if elements and auto_save:
                             self.save(elements[0], identifier or selector)
 
                 return self.__handle_elements(elements)