Only choose a (3, 1) cmap table for TrueType fonts that have an encoding specified (issue 6410)

For (1, 0) cmaps, we have two different codepaths depending on whether the font has/hasn't got an encoding. But with (3, 1) cmaps we don't have a good fallback when the encoding is missing, hence this patch changes `readCmapTable` to only choose a (3, 1) cmap table if the font is non-symbolic *and* an encoding exists. Without this, we'll not be able to successfully create a working glyph map for some TrueType fonts with (3, 1) cmap tables.

Fixes 6410.

test/pdfs/.gitignore

@@ -98,6 +98,7 @@
 !issue5747.pdf
 !issue6099.pdf
 !issue6336.pdf
+!issue6410.pdf
 !gradientfill.pdf
 !bug903856.pdf
 !bug850854.pdf

test/test_manifest.json

@@ -1086,6 +1086,13 @@
       "link": true,
       "type": "eq"
     },
+    {  "id": "issue6410",
+       "file": "pdfs/issue6410.pdf",
+       "md5": "fd5c5898d5b9754bb546724b7d31bf59",
+       "rounds": 1,
+       "link": false,
+       "type": "eq"
+    },
     {  "id": "issue6413",
        "file": "pdfs/issue6413.pdf",
        "md5": "08926ac7a46e27a4abbb31256b3a7b29",