feat[lang]: reject implements for empty interfaces

`implements: <interface>` for an empty interface likely indicates a bug
in user code, reject it.

tests/functional/syntax/modules/test_implements.py

@@ -1,4 +1,7 @@
+import pytest
+
 from vyper.compiler import compile_code
+from vyper.exceptions import StructureException
 
 
 def test_implements_from_vyi(make_input_bundle):
@@ -49,3 +52,21 @@ def foo():  # implementation
     input_bundle = make_input_bundle({"some_interface.vyi": vyi, "lib1.vy": lib1, "lib2.vy": lib2})
 
     assert compile_code(main, input_bundle=input_bundle) is not None
+
+
+def test_implements_empty_vyi(make_input_bundle, tmp_path):
+    vyi = ""
+    input_bundle = make_input_bundle({"some_interface.vyi": vyi})
+    main = """
+import some_interface
+
+implements: some_interface
+    """
+    with pytest.raises(StructureException) as e:
+        _ = compile_code(main, input_bundle=input_bundle)
+
+    vyi_path = (tmp_path / "some_interface.vyi").as_posix()
+    assert (
+        e.value._message
+        == f"Tried to implement `{vyi_path}`, but it has no functions to implement!"
+    )

vyper/semantics/types/module.py

@@ -112,6 +112,10 @@ def _ctor_modifiability_for_call(self, node: vy_ast.Call, modifiability: Modifia
     def validate_implements(
         self, node: vy_ast.ImplementsDecl, functions: dict[ContractFunctionT, vy_ast.VyperNode]
     ) -> None:
+        if len(self.functions) == 0:
+            msg = f"Tried to implement `{self}`, but it has no functions to implement!"
+            raise StructureException(msg, node)
+
         # only external functions can implement interfaces
         fns_by_name = {fn_t.name: fn_t for fn_t in functions.keys()}