Allow finding for empty strings with wstr::find

I hit this temporarily in a test; it seems reasonable to allow this.
std::str does too.

src/common.rs

@@ -1632,10 +1632,13 @@ pub fn restore_term_foreground_process_group_for_exit() {
 #[allow(unused)]
 // This function is unused in some configurations/on some platforms
 fn slice_contains_slice<T: Eq>(a: &[T], b: &[T]) -> bool {
-    a.windows(b.len()).any(|aw| aw == b)
+    subslice_position(a, b).is_some()
 }
 
 pub fn subslice_position<T: Eq>(a: &[T], b: &[T]) -> Option<usize> {
+    if b.is_empty() {
+        return Some(0);
+    }
     a.windows(b.len()).position(|aw| aw == b)
 }
 

src/wchar_ext.rs

@@ -245,11 +245,7 @@ pub trait WExt {
 
     /// Returns the index of the first match against the provided substring or `None`.
     fn find(&self, search: impl AsRef<[char]>) -> Option<usize> {
-        fn inner(lhs: &[char], rhs: &[char]) -> Option<usize> {
-            lhs.windows(rhs.len()).position(|window| window == rhs)
-        }
-
-        inner(self.as_char_slice(), search.as_ref())
+        subslice_position(self.as_char_slice(), search.as_ref())
     }
 
     /// Replaces all matches of a pattern with another string.