CouchDB Map syntax - mapreduce

Question

I have documents in CouchDB (v. 2.1.1) as follows:
{
"xyz": "a",
"abc": "def"
},
{
"xyz": "a",
"ghi": "jkl"
},
{
"xyz": "a",
"mno": "pqr"
},
{
"xyz": "a",
"stu": "vwx"
},
{
"xyz": "a",
"bcd": 1000
}
If I run a simple map function, for example:
function (doc) {
if (doc.xyz ){
emit(doc.xyz, doc.abc);}}
I get:
{
"id": "4c3406a1d92942b4fb10d1314e0061a9",
"key": "a",
"value": "def"
},
{
"id": "4c3406a1d92942b4fb10d1314e006ccf",
"key": "a",
"value": null
},
{
"id": "4c3406a1d92942b4fb10d1314e00787f",
"key": "a",
"value": null
},
{
"id": "4c3406a1d92942b4fb10d1314e00871e",
"key": "a",
"value": null
},
{
"id": "4c3406a1d92942b4fb10d1314e00906a",
"key": "a",
"value": null
}
I want to try and eliminate the 'null' outputs.
I am looking at having a CouchDB database with many small documents containing small snippets of information rather than having larger documents containing much more information per document.
My question is, is my document design a good one and if so how do I get just what I am looking for rather than rows of 'nulls'. If my storage design is not ideal, what kind of design should I be looking at to simplify the output given my plan to have many small 'docs'.
EDIT:
Having looked at possible answers, I have decided that having numerous small documents as I described in my question is not giving me the kind of benefit I imangined they would.
I was unable to get a satisfactory solution to the map function to get readable answers.
However, I investigated the 'Mango' query system available in recent updates of CouchDB and I was able using these queries to get acceptable output from a database like my supplied one.
This is what I did:
curl -X POST http://admin:123#127.0.0.1:5984/ptn/_find -d '{"selector": {"$or": [{"abc": {"$gt": null}},{"ghi": {"$gt": null}}]},"fields": ["abc","ghi"]}' -H "Content-Type:application/json"
Un-minified:
{
"selector": {
"$or": [
{
"abc": {
"$gt": null
}
},
{
"ghi": {
"$gt": null
}
}
]
},
"fields": [
"abc",
"ghi"
]
}
The output:
{"docs":[
{"abc":"def"},
{"ghi":"jkl"}
]
.....
A concise answer.
Sorting can be done but sorted fields must be indexed. Indexing is in any case advised for larger data sets.
Reference:
http://docs.couchdb.org/en/2.1.1/api/database/find.html
As my question required a map function, this perhaps cannot be regarded as a valid answer but for me it is an answer. I have tried the 'Mango' query system a little on other databases and it seems to be more useful/powerful than I thought is was although it offers no means of totaling etc.