Do not convert non-truthy dapi return values to empty string

* src/dapi/DataApiManager.js (getDataExpansion): Explicit undefined
  check before triggering default (empty string).
* test/dapi/DataApiManagerTest.js: Add associated test cases.

src/dapi/DataApiManager.js

@@ -679,7 +679,10 @@ module.exports = Class( 'DataApiManager' )
             var param = map[ field ],
                 fdata = [];
 
-            fdata[ index ] = ( data[ param ] || '' );
+            fdata[ index ] = ( data[ param ] !== undefined )
+                ? data[ param ]
+                : '';
+
             update[ field ] = fdata;
         }
 

test/dapi/DataApiManagerTest.js

@@ -54,6 +54,45 @@ describe( 'DataApiManager', () =>
             .setApis( { [type]: { type: type } } )
             .getApiData( type, {}, ()=>{}, name, index, {}, fail );
     } );
+
+
+    // TODO: This doesn't test everything; see commit that introduced it.
+    [
+        {
+            map:      { out: 'in' },
+            fdata:    [ { in: 'foo' } ],
+            expected: { out: [ 'foo' ] },
+        },
+
+        // retain booleans (in particular, don't convert `false' to an empty
+        // string
+        {
+            map:      { out: 'in' },
+            fdata:    [ { in: true } ],
+            expected: { out: [ true ] },
+        },
+        {
+            map:      { out: 'in' },
+            fdata:    [ { in: false } ],
+            expected: { out: [ false ] },
+        },
+    ].forEach( ( { map, fdata, expected } ) =>
+    {
+        it( 'generates update from data expansion', () =>
+        {
+            const name  = 'fooname';
+            const index = 0;
+
+            const sut = Sut( createStubDapiFactory( {} ) );
+
+            sut.setFieldData( name, index, fdata, 'in', '' );
+
+            const bucket = { getDataByName: () => [ fdata[ 0 ].in ] };
+
+            expect( sut.getDataExpansion( name, index, bucket, map, true, {} ) )
+                .to.deep.equal( expected );
+        } )
+    } );
 } );