Invalid size set when map is not added to document

Parsing of an empty string with parseFloat resulted in NaN.

test/spec/ol/map.test.js

@@ -687,6 +687,7 @@ describe('ol.Map', function () {
   });
 
   describe('#setTarget', function () {
+    /** @type {Map|undefined} */
     let map;
 
     beforeEach(function () {
@@ -696,6 +697,12 @@ describe('ol.Map', function () {
       expect(map.handleResize_).to.be.ok();
     });
 
+    describe('map with target not attached to dom', function () {
+      it('has undefined as size with target not in document', function () {
+        expect(map.getSize()).to.be(undefined);
+      });
+    });
+
     describe('call setTarget with null', function () {
       it('unregisters the viewport resize listener', function () {
         map.setTarget(null);