Add regression test for issue 99173

Adds a test for nested proc-macro calls where the inner macro returns an
empty TokenStream.
This previously caused an ICE in rustc_resolve.

tests/ui/proc-macro/auxiliary/nested-empty-proc-macro.rs

@@ -0,0 +1,18 @@
+// Auxiliary proc-macro for issue #99173
+// Tests that nested proc-macro calls with empty output don't cause ICE
+
+extern crate proc_macro;
+
+use proc_macro::TokenStream;
+
+// This macro returns an empty TokenStream
+#[proc_macro]
+pub fn ignore(_input: TokenStream) -> TokenStream {
+    TokenStream::new()
+}
+
+// This macro generates code that calls the `ignore` macro
+#[proc_macro]
+pub fn outer_macro(_input: TokenStream) -> TokenStream {
+    "nested_empty_proc_macro::ignore!(42)".parse().unwrap()
+}

tests/ui/proc-macro/nested-empty-proc-macro.rs

@@ -0,0 +1,12 @@
+//@ check-pass
+//@ proc-macro: nested-empty-proc-macro.rs
+
+// Regression test for issue #99173
+// Tests that nested proc-macro calls where the inner macro returns
+// an empty TokenStream don't cause an ICE.
+
+extern crate nested_empty_proc_macro;
+
+fn main() {
+    nested_empty_proc_macro::outer_macro!(1 * 2 * 3 * 7);
+}