Require there to be at least one item.

Fixes WebAssembly#67

imports.md

@@ -77,8 +77,11 @@ containing only this pollable.</p>
 interest, and waits until one or more of the events is ready for I/O.</p>
 <p>The result <code>list&lt;u32&gt;</code> contains one or more indices of handles in the
 argument list that is ready for I/O.</p>
-<p>If the list contains more elements than can be indexed with a <code>u32</code>
-value, this function traps.</p>
+<p>This function traps if either:</p>
+<ul>
+<li>the list is empty, or:</li>
+<li>the list contains more elements than can be indexed with a <code>u32</code> value.</li>
+</ul>
 <p>A timeout can be implemented by adding a pollable from the
 wasi-clocks API to the list.</p>
 <p>This function does not return a <code>result</code>; polling in itself does not

wit/poll.wit

@@ -27,8 +27,9 @@ interface poll {
     /// The result `list<u32>` contains one or more indices of handles in the
     /// argument list that is ready for I/O.
     ///
-    /// If the list contains more elements than can be indexed with a `u32`
-    /// value, this function traps.
+    /// This function traps if either:
+    /// - the list is empty, or:
+    /// - the list contains more elements than can be indexed with a `u32` value.
     ///
     /// A timeout can be implemented by adding a pollable from the
     /// wasi-clocks API to the list.