evaluate: throw distinct error if map exists but contains no objects

nft would throw misleading error in case map exists but doesn't contain
expected objects.

nft add rule filter in ct helper set tcp dport map @foo
Error: Expression is not a map
add rule filter in ct helper set tcp dport map @foo
                                               ^^^^
nft list table filter
table ip filter {
        map foo {
                type inet_service : ifname
        }
...

clarify this.

Reported-by: Christian Göttsche <cgzones@googlemail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>

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