Fix formatting failures on Windows

When newline_style is set to Windows, an empty line inside of a macro
results in `\r` being passed to the `fold()` in `MacroBranch::rewrite()`.

`\r` is technically not an empty string, so we try to indent it, leaving
trailing whitespaces behind, even though that was not intended
(as far as I can see).

This commit replaces the `!l.is_empty()` check with calling
`is_empty_line()`, since trying to indent any whitespace-only string
will probably result in problematic trailing whitespaces.

Fixes: #2810

src/macros.rs

@@ -1347,7 +1347,7 @@ impl MacroBranch {
             .fold(
                 (String::new(), true),
                 |(mut s, need_indent), (i, (kind, ref l))| {
-                    if !l.is_empty()
+                    if !is_empty_line(l)
                         && need_indent
                         && !new_body_snippet.is_line_non_formatted(i + 1)
                     {

tests/target/issue-2810.rs

@@ -0,0 +1,14 @@
+// rustfmt-newline_style: Windows
+
+#[macro_export]
+macro_rules! hmmm___ffi_error {
+    ($result:ident) => {
+        pub struct $result {
+            success: bool,
+        }
+
+        impl $result {
+            pub fn foo(self) {}
+        }
+    };
+}