buildifier sorts strings of lists inconsistently when a comment appears in the middle of the list

I noticed that a line with only a comment in it will break any sorting of AssignExprs when there is a comment before the assignment, but if the "keep sorted" comment is the first element in the list, then it will sort the list. Example:

```
list1 = [
"foo",
"bar",
"buzz",
]

# buildifier: keep sorted
list2 = [
"bar",
"buzz",
"foo",
]

# this will not be sorted:
# buildifier: keep sorted
list3 = [
"foo",
"bar",
"buzz",
# "blah",
"abc123",
]

# this WILL be sorted:
list4 = [
# buildifier: keep sorted
"foo",
"bar",
"buzz",
# "blah",
"abc123",
]
```

buildifier will sort `list4` but not `list3`:

```
❯ buildifier -mode=diff -lint=warn example.bzl
--- example.bzl 2026-02-05 16:06:51
+++ /var/folders/m1/5018nxw5655fd55ql3xqqpjm0000gn/T/buildifier-tmp-3189288486 2026-02-05 16:10:37
@@ -24,9 +24,9 @@
# this WILL be sorted:
list4 = [
# buildifier: keep sorted
- "foo",
"bar",
"buzz",
+ "foo",
# "blah",
"abc123",
]
exit status 1
```

Reproduce the behavior with the shown example.bzl and `buildifier -mode=diff -lint=warn example.bzl`, comparing list3 and list4. Trace how buildifier handles keep-sorted comments and intervening comment-only lines, then verify that both list forms are sorted consistently without changing unrelated output.

Written by the indexing model from the issue text.