[release-branch.go1.11] encoding/json: fix UnmarshalTypeError without field and struct values

Updates #26444
Updates #27275
Fixes #27318

Change-Id: I9e8cbff79f7643ca8964c572c1a98172b6831730
GitHub-Last-Rev: 7eea2158b67ccab34b45a21e8f4289c36de02d93
GitHub-Pull-Request: golang/go#26719
Reviewed-on: https://go-review.googlesource.com/126897


Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-on: https://go-review.googlesource.com/138178
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>