Optimize substitute method (pyccel#1938)

### Description
This PR addresses an optimization for the substitute method in the
PyccelAstNode class. The current implementation does not check if
original and replacement are empty, leading to unnecessary recursion
over the code. This update adds a condition to return immediately if
original is empty (and consequenty replacement), thereby improving the
performance of the method.

---------

Co-authored-by: Emily Bourne <[email protected]>

Author: jalalium

CHANGELOG.md

@@ -27,6 +27,7 @@ All notable changes to this project will be documented in this file.
 -   #1895 : Add Python support for dict initialisation with `dict()`.
 -   #1886 : Add Python support for dict method `pop()`.
 -   #1936 : Add missing C output for inline decorator example in documentation
+-   #1937 : Optimise `pyccel.ast.basic.PyccelAstNode.substitute` method.
 -   \[INTERNALS\] Added `container_rank` property to `ast.datatypes.PyccelType` objects.
 -   \[DEVELOPER\] Added an improved traceback to the developer-mode errors for errors in function calls.
 

pyccel/ast/basic.py

@@ -279,6 +279,11 @@ def substitute(self, original, replacement, excluded_nodes = (), invalidate = Tr
             return
         self._recursion_in_progress = True
 
+        if not original:
+            assert not replacement
+            self._recursion_in_progress = False
+            return
+
         if iterable(original):
             assert iterable(replacement)
             assert len(original) == len(replacement)