`Pointer::push_back`: Handle empty keys

[wngr]

Hey!

When using Pointer::push_back with empty strings as keys the count was incremented, but the actual inner pointer value was not updated.

This PR fixes that.

I'm not sure why the check !self.inner.ends_with('/') was present in the first place.

[asmello]

Adding some context.

Per RFC 6901§5:

   For example, given the JSON document

   {
      "foo": ["bar", "baz"],
      "": 0,
      "a/b": 1,
      "c%d": 2,
      "e^f": 3,
      "g|h": 4,
      "i\\j": 5,
      "k\"l": 6,
      " ": 7,
      "m~n": 8
   }

   The following JSON strings evaluate to the accompanying values:

    ""           // the whole document
    "/foo"       ["bar", "baz"]
    "/foo/0"     "bar"
    "/"          0
    "/a~1b"      1
    "/c%d"       2
    "/e^f"       3
    "/g|h"       4
    "/i\\j"      5
    "/k\"l"      6
    "/ "         7
    "/m~0n"      8

Note how / is a valid JSON pointer that evaluates to a root field keyed by an empty string (""), and not the whole document (which is represented as "", confusingly enough). Unlike with UNIX-style paths, every forward slash character in JSON pointers is significant and cannot be ignored when constructing paths with Pointer::push_back.

[chanced]

Yea, the "/" being a valid, non-root path confused me when I first wrote it.

The check for "/" must have been an artifact I forgot to remove when I realized "" was actually the root.

Sorry about that and thank you for the fix!

[chanced]

This should be available under v0.4.5. Thanks y'all.