build: sort.py: ignore empty files

Currently it adds a newline to empty files.

Before:

    $ : >foo.profile
    $ contrib/sort.py foo.profile
    sort.py: checking 1 profile(s)...
    foo.profile:(fixed whitespace)
    [ Fixed ] foo.profile
    $ od -A n -t x1 foo.profile
     0a

After:

    $ : >foo.profile
    $ contrib/sort.py foo.profile
    sort.py: checking 1 profile(s)...
    $

This amends commit c222b7f ("build: sort.py: fix whitespace in entire
profile (#6593)", 2024-12-28).

contrib/sort.py

@@ -76,7 +76,11 @@ def sort_protocol(original_protocols):
 
 def check_profile(filename, overwrite):
     with open(filename, "r+") as profile:
-        lines = profile.read().split("\n")
+        original_profile_str = profile.read()
+        if not original_profile_str:
+            return
+
+        lines = original_profile_str.split("\n")
         was_fixed = False
         fixed_profile = []
         for lineno, original_line in enumerate(lines, 1):