extension: Print empty entry if there is no extension

Because we now count the extension including the ".", we print an
empty entry.

This makes e.g.

```fish
set -l base (path change-extension '' $somefile)
set -l ext (path extension $somefile)
echo $base$ext
```

reconstruct the filename, and makes it easier to deal with files with
no extension.

src/builtins/path.cpp

@@ -593,7 +593,12 @@ static int path_extension(parser_t &parser, io_streams_t &streams, int argc, con
     while (const wcstring *arg = aiter.nextstr()) {
         auto pos = find_extension(*arg);
 
-        if (!pos) continue;
+        if (!pos) {
+            // If there is no extension the extension is empty.
+            // This is unambiguous because we include the ".".
+            path_out(streams, opts, L"");
+            continue;
+        }
 
         wcstring ext = arg->substr(*pos);
         if (opts.quiet && !ext.empty()) {

tests/checks/path.fish

@@ -4,20 +4,24 @@
 # Extension - for figuring out the file extension of a given path.
 path extension /
 or echo None
+# CHECK:
 # CHECK: None
 
 # No extension
 path extension /.
 or echo Filename is just a dot, no extension
+# CHECK:
 # CHECK: Filename is just a dot, no extension
 
 # No extension - ".foo" is the filename
 path extension /.foo
 or echo None again
+# CHECK:
 # CHECK: None again
 
 path extension /foo
 or echo None once more
+# CHECK:
 # CHECK: None once more
 path extension /foo.txt
 and echo Success
@@ -25,17 +29,21 @@ and echo Success
 # CHECK: Success
 path extension /foo.txt/bar
 or echo Not even here
+# CHECK:
 # CHECK: Not even here
 path extension . ..
 or echo No extension
+# CHECK:
 # CHECK: No extension
 path extension ./foo.mp4
 # CHECK: .mp4
 path extension ../banana
+# CHECK:
 # nothing, status 1
 echo $status
 # CHECK: 1
 path extension ~/.config
+# CHECK:
 # nothing, status 1
 echo $status
 # CHECK: 1