Do not increment prefix upper bound if there are no tokens.

This change fixes a bug wherein `Glob::partition` panics if the `Glob`
is empty and has no tokens. In this case,
`invariant_text_prefix_upper_bound` returns `1`, but should return `0`.

src/lib.rs

@@ -1986,6 +1986,13 @@ mod tests {
         assert_eq!(format!("{}", glob), "**");
     }
 
+    #[test]
+    fn partition_empty_glob() {
+        let (prefix, glob) = Glob::new("").unwrap().partition();
+        assert_eq!(prefix, Path::new(""));
+        assert_eq!(format!("{}", glob), "");
+    }
+
     #[test]
     fn repartition_glob_with_variant_tokens() {
         let (prefix, glob) = Glob::new("/root/**/file.ext").unwrap().partition();
@@ -1997,6 +2004,14 @@ mod tests {
         assert_eq!(format!("{}", glob), "**/file.ext");
     }
 
+    #[test]
+    fn repartition_empty_glob() {
+        let (_, glob) = Glob::new("").unwrap().partition();
+        let (prefix, glob) = glob.partition();
+        assert_eq!(prefix, Path::new(""));
+        assert_eq!(format!("{}", glob), "");
+    }
+
     #[test]
     fn query_glob_has_root() {
         assert!(Glob::new("/root").unwrap().has_root());

src/token/variance.rs

@@ -1,5 +1,6 @@
 use itertools::Itertools as _;
 use std::borrow::Cow;
+use std::cmp;
 use std::collections::VecDeque;
 use std::ops::{Add, Mul};
 
@@ -556,7 +557,7 @@ where
             },
         }
     }
-    m + 1
+    m + cmp::min(m, 1)
 }
 
 /// Returns `true` if the token tree is exhaustive.