Add a test with an empty crate name.

This error was untested.

tests/ui/attributes/crate-name-empty.rs

@@ -0,0 +1,5 @@
+// Ensure we reject `#![crate_name = ""]`.
+
+#![crate_name = ""] //~ ERROR crate name must not be empty
+
+fn main() {}

tests/ui/attributes/crate-name-empty.stderr

@@ -0,0 +1,8 @@
+error: crate name must not be empty
+  --> $DIR/crate-name-empty.rs:3:1
+   |
+LL | #![crate_name = ""]
+   | ^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+