Fix the Skylark specification
Return statements can exist without an expression.
@@ -1997,7 +1997,7 @@ A `return` statement ends the execution of a function and returns a
value to the caller of the function.
```text
-ReturnStmt = 'return' Expression .
+ReturnStmt = 'return' [Expression] .
```
A return statement may have zero, one, or more
@@ -3192,7 +3192,7 @@ SmallStmt = ReturnStmt
| LoadStmt
.
-ReturnStmt = 'return' Expression .
+ReturnStmt = 'return' [Expression] .
BreakStmt = 'break' .
ContinueStmt = 'continue' .
PassStmt = 'pass' .
