Fix empty view setup if configured before parent view

Before this change, if you were to assign a data source’s emptyView
*before* you assigned its tableView or collectionView, the emptyView
would not be displayed if there were no items. This was because the
emptyView was not [properly updated since at the time of the assignment
its parent view would be nil][1].

Before:

```objc
dataSource.emptyView = [MyAwesomeView new];
// other stuff
dataSource.tableView = self.tableView;
// if dataSource has no items, MyAwesomeView will never appear :(
```

After:

```objc
dataSource.emptyView = [MyAwesomeView new];
// other stuff
dataSource.tableView = self.tableView;
// if dataSource has no items, MyAwesomeView will appear in all of its
// glory :)
```

[1]: https://github.com/splinesoft/SSDataSources/blob/2321bc79e9a9b2a537ffe8e806b08fbc021b13e2/SSDataSources/SSBaseDataSource.m#L248-L250

Author: andrewsardone

Example/ExampleSSDataSourcesTests/SSBaseDataSourceTests.m

@@ -204,4 +204,21 @@ - (void)testBasicEmptyViewVisibility
     expect(arrayDataSource.emptyView.hidden).to.beFalsy();
 }
 
+- (void)testEmptyViewSetUpIsNotDependentOnParentViewConfiguration
+{
+    // The intent is to ensure that you can set up your emptyView *before* you
+    // assign the data source’s table or collection view.
+    SSArrayDataSource *arrayDataSource;
+
+    arrayDataSource = [[SSArrayDataSource alloc] initWithItems:@[]];
+    arrayDataSource.emptyView = [UIView new];
+    arrayDataSource.tableView = tableView;
+    expect(arrayDataSource.emptyView.hidden).to.beFalsy();
+
+    arrayDataSource = [[SSArrayDataSource alloc] initWithItems:@[]];
+    arrayDataSource.emptyView = [UIView new];
+    arrayDataSource.collectionView = collectionView;
+    expect(arrayDataSource.emptyView.hidden).to.beFalsy();
+}
+
 @end

SSDataSources/SSBaseDataSource.m

@@ -104,6 +104,8 @@ - (void)setTableView:(UITableView *)tableView {
     if (tableView) {
         tableView.dataSource = self;
     }
+
+    [self _updateEmptyView];
 }
 
 - (void)setCollectionView:(UICollectionView *)collectionView {
@@ -112,6 +114,8 @@ - (void)setCollectionView:(UICollectionView *)collectionView {
     if (collectionView) {
         collectionView.dataSource = self;
     }
+
+    [self _updateEmptyView];
 }
 
 #pragma mark - UITableViewDataSource