Python: fix ChangeImport leaving stray leading newline when import is the only statement (#7433)

When `ChangeImport` replaced the only statement in a file, the
scheduled `AddImport` pass ran against an empty statement list and fell
through to the "inserting after existing imports" branch, prefixing the
new import with `\n` and producing `\nfrom collections.abc import Callable\n`.

Handle the empty-statement-list case explicitly: the new import becomes
the sole statement with empty prefix; the trailing newline stays in
`cu.eof`.

Author: knutwannheden

rewrite-python/rewrite/src/rewrite/python/add_import.py

@@ -260,7 +260,13 @@ def _add_import(self, cu: CompilationUnit) -> CompilationUnit:
                 break  # Stop after we've passed the import section
 
         # Insert the new import at the padding level
-        if insert_idx == 0 and padded_stmts:
+        if not padded_stmts:
+            # Empty file (e.g., a prior remove-import emptied the statement
+            # list). The new import becomes the sole statement with empty
+            # prefix; any trailing newline lives in cu.eof.
+            new_padded = JRightPadded(new_import.replace(prefix=Space.EMPTY), Space.EMPTY, Markers.EMPTY)
+            padded_stmts.append(new_padded)
+        elif insert_idx == 0:
             first = padded_stmts[0]
             first_prefix = first.element.prefix
             if first_prefix.whitespace and first_prefix.whitespace.startswith('\n'):

rewrite-python/rewrite/tests/recipes/test_change_import.py

@@ -46,6 +46,20 @@ def test_change_from_import_module_and_name(self):
             )
         )
 
+    def test_change_import_only_statement(self):
+        """Import is the only statement in the file — no leading newline in output."""
+        spec = RecipeSpec(recipe=ChangeImport(
+            old_module='typing',
+            old_name='Callable',
+            new_module='collections.abc',
+        ))
+        spec.rewrite_run(
+            python(
+                "from typing import Callable\n",
+                "from collections.abc import Callable\n",
+            )
+        )
+
     def test_change_direct_import(self):
         """Change: import os -> import pathlib"""
         spec = RecipeSpec(recipe=ChangeImport(