fix(builtin): provide proper error if npm_package_bin has no outs (#2557)

Author: alexeagle

internal/node/npm_package_bin.bzl

@@ -48,7 +48,7 @@ def _inputs(ctx):
 def _impl(ctx):
     if ctx.attr.output_dir and ctx.outputs.outs:
         fail("Only one of output_dir and outs may be specified")
-    if not ctx.attr.output_dir and not ctx.outputs.outs and not ctx.attr.stdout:
+    if not ctx.attr.output_dir and not len(ctx.outputs.outs) and not ctx.attr.stdout:
         fail("One of output_dir, outs or stdout must be specified")
 
     args = ctx.actions.args()