feat: allow empty blocks in parser and fix native call slot assertion

Add support for empty block bodies in the Wren compiler by early-returning
from `finishBlock` when a right brace is immediately matched. This permits
syntactically valid constructs like `{}`, `if (true) {}`, and empty function
or method bodies.

Fix a copy-paste error in `wrenGetArgumentDouble` where the assertion
checked `nativeCallSlot` and `nativeCallNumArgs` instead of the correct
`foreignCallSlot` and `foreignCallNumArgs` fields.

Add test cases covering standalone empty blocks, empty blocks in if/else
statements, and empty function and method bodies returning null.

src/wren_compiler.c

@@ -1192,6 +1192,9 @@ static void patchJump(Compiler* compiler, int offset)
 // Parses a block body, after the initial "{" has been consumed.
 static void finishBlock(Compiler* compiler)
 {
+  // Empty blocks do nothing.
+  if (match(compiler, TOKEN_RIGHT_BRACE)) return;
+
   for (;;)
   {
     definition(compiler);

src/wren_vm.c

@@ -1288,9 +1288,9 @@ void wrenDefineMethod(WrenVM* vm, const char* className,
 
 double wrenGetArgumentDouble(WrenVM* vm, int index)
 {
-  ASSERT(vm->nativeCallSlot != NULL, "Must be in foreign call.");
+  ASSERT(vm->foreignCallSlot != NULL, "Must be in foreign call.");
   ASSERT(index >= 0, "index cannot be negative.");
-  ASSERT(index < vm->nativeCallNumArgs, "Not that many arguments.");
+  ASSERT(index < vm->foreignCallNumArgs, "Not that many arguments.");
 
   // + 1 to shift past the receiver.
   // TODO: Check actual value type first.