Use post table when Encoding has only Differences

Fixes #13107
In the issue, some TrueType glyph names have the format `uniXXXX`.
Font's `Encoding` dictionary has the entry `Differences` but no
`BaseEncoding`. `uniXXXX` names are converted to glyph indices
using font's `post` table but currently that is done only when
`BaseEncoding` exists. We must enable the conversion also when only
`Differences` exists.

test/pdfs/.gitignore

@@ -158,6 +158,7 @@
 !issue6019.pdf
 !issue6621.pdf
 !issue6286.pdf
+!issue13107_reduced.pdf
 !issue1055r.pdf
 !issue11713.pdf
 !issue1293r.pdf

test/test_manifest.json

@@ -595,6 +595,12 @@
        "link": false,
        "type": "eq"
     },
+    {  "id": "issue13107",
+      "file": "pdfs/issue13107_reduced.pdf",
+      "md5": "13c8d12223dec2752d3c90d250db32b7",
+      "rounds": 1,
+      "type": "eq"
+    },
     {  "id": "bug1186827",
        "file": "pdfs/bug1186827.pdf",
        "md5": "6c5526ae1a9d66cb517153001afc196e",