Add Directory::is_empty for completeness

src/directory.rs

@@ -27,6 +27,8 @@ pub struct Directory {
 }
 
 impl Directory {
+    /// Create a directory in an initial state without entries. Note that an empty directory can
+    /// not be encoded in a file, it must contain at least one entry.
     pub fn empty() -> Self {
         Directory {
             entries: BTreeMap::new(),
@@ -72,6 +74,12 @@ impl Directory {
         self.entries.len()
     }
 
+    /// Check if there are any entries in this directory. Note that an empty directory can not be
+    /// encoded in the file, it must contain at least one entry.
+    pub fn is_empty(&self) -> bool {
+        self.entries.is_empty()
+    }
+
     /// Get the pointer to the next IFD, if it was defined.
     pub fn next(&self) -> Option<IfdPointer> {
         self.next_ifd.map(|n| IfdPointer(n.get()))