terminal: ignore empty cell clears

Screen.clearCells accepted a slice but its runtime safety validation
indexed the first and last elements unconditionally. Passing an empty
range therefore panicked before the otherwise valid no-op clear.

Return immediately for an empty slice so validation and managed-memory
bookkeeping only run when there are cells to clear.

src/terminal/Screen.zig

@@ -1546,6 +1546,8 @@ pub fn clearCells(
     row: *Row,
     cells: []Cell,
 ) void {
+    if (cells.len == 0) return;
+
     // This whole operation does unsafe things, so we just want to assert
     // the end state.
     page.pauseIntegrityChecks(true);
@@ -4084,6 +4086,18 @@ test "Screen clearRows active styled line" {
     try testing.expectEqualStrings("", str);
 }
 
+test "Screen clearCells empty range" {
+    const testing = std.testing;
+
+    var s = try Screen.init(testing.allocator, .default);
+    defer s.deinit();
+
+    const page = s.cursor.page_pin.node.page();
+    const row = s.cursor.page_row;
+    const cells = page.getCells(row);
+    s.clearCells(page, row, cells[0..0]);
+}
+
 test "Screen clearRows protected" {
     const testing = std.testing;
     const alloc = testing.allocator;