Re-factor the handling of *empty* Name-instances (PR 13612 follow-up)

When working on PR 13612, I mostly prioritized a simple solution that didn't require touching a lot of code. However, while working on PR 13735 I started to realize that the static `Name.empty` construction really wasn't a good idea.

In particular, having a special `Name`-instance where the `name`-property isn't actually a String is confusing (to put it mildly) and can easily lead to issues elsewhere. The only reason for not simply allowing the `name`-property to be an *empty* string, in PR 13612, was to avoid having to touch a lot of existing code. However, it turns out that this is only limited to a few methods in the `PartialEvaluator` and a few of the `BaseLocalCache`-implementations, all of which can be easily re-factored to handle *empty* `Name`-instances.

All-in-all, I think that this patch is even an *overall* improvement since we're now validating (what should always be) `Name`-data better in the `PartialEvaluator`.
This is what I ought to have done from the start, sorry about the code churn here!

src/core/parser.js

@@ -1115,7 +1115,6 @@ class Lexer {
       warn(`Name token is longer than allowed by the spec: ${strBuf.length}`);
     } else if (strBuf.length === 0) {
       warn("Name token is empty.");
-      return Name.empty;
     }
     return Name.get(strBuf.join(""));
   }