I find it difficult to accept that such a huge chunk of the galaxy would be completely unknown for the Empire, or anyone in the galaxy, for that matter. Even if every particular system was not charted in that region, they should have a rough outline from scans, telescopes etc.

For me, two tweaks would alleviate the map issue.

R2’s map would be a roughly COMPLETE MAP of the galaxy with no “hole” in it. The region highlighted in BB8’s map bit would provide greater detail in that specific region, including the final section of the orange “route” to Luke, but the surrounding area wouldn’t be completely blank.
C-3PO’s line, “it matches no charted system on record” is modified to “it matches no charted systems on record”, because the obtained map clearly shows a multitude of star systems, in fact, a whole region of the galaxy, not just a single one.

In my head canon, the orange “route” is a map compiled from Luke’s rumored or reported locations and/or the known locations of notable Jedi sites, and that is the “map”, not just the location of a specific star system in the galaxy.