cli: simplify check for non-empty revisions with/without "all:"

If "all:" is specified, an empty set should be allowed within that expression.
So the additional check we need here is to ensure that the resulting set is not
empty.

cli/src/cli_util.rs

@@ -1658,9 +1658,6 @@ pub fn resolve_multiple_nonempty_revsets_default_single(
     let mut all_commits = IndexSet::new();
     for revision_str in revisions {
         let commits = workspace_command.resolve_revset_default_single(revision_str)?;
-        if commits.is_empty() {
-            return Err(user_error("Empty revision set"));
-        }
         for commit in commits {
             let commit_hash = short_commit_hash(commit.id());
             if !all_commits.insert(commit) {
@@ -1670,7 +1667,11 @@ pub fn resolve_multiple_nonempty_revsets_default_single(
             }
         }
     }
-    Ok(all_commits)
+    if all_commits.is_empty() {
+        Err(user_error("Empty revision set"))
+    } else {
+        Ok(all_commits)
+    }
 }
 
 pub fn update_working_copy(

cli/src/commands/new.rs

@@ -96,10 +96,6 @@ Please use `jj new 'all:x|y'` instead of `jj new --allow-large-revsets x y`.",
         ));
     }
     let mut workspace_command = command.workspace_helper(ui)?;
-    assert!(
-        !args.revisions.is_empty(),
-        "expected a non-empty list from clap"
-    );
     let target_commits =
         resolve_multiple_nonempty_revsets_default_single(&workspace_command, &args.revisions)?
             .into_iter()