Fix nullability annotations in NITableViewModel (jverkoey#701)

These were failing with the following error:

```
nimbus/src/models/src/NITableViewModel.m:61:16: error: null passed to a callee that requires a non-null argument [-Werror,-Wnonnull]
  return [self initWithDelegate:nil];
```

Author: featherless

src/models/src/NITableViewModel.h

@@ -54,10 +54,10 @@ typedef enum {
 #pragma mark Creating Table View Models
 
 // Designated initializer.
-- (nonnull id)initWithDelegate:(nonnull id<NITableViewModelDelegate>)delegate;
-- (nonnull id)initWithListArray:(nonnull NSArray *)sectionedArray delegate:(nonnull id<NITableViewModelDelegate>)delegate;
+- (nonnull id)initWithDelegate:(nullable id<NITableViewModelDelegate>)delegate NS_DESIGNATED_INITIALIZER;
+- (nonnull id)initWithListArray:(nonnull NSArray *)sectionedArray delegate:(nullable id<NITableViewModelDelegate>)delegate;
 // Each NSString in the array starts a new section. Any other object is a new row (with exception of certain model-specific objects).
-- (nonnull id)initWithSectionedArray:(nonnull NSArray *)sectionedArray delegate:(nonnull id<NITableViewModelDelegate>)delegate;
+- (nonnull id)initWithSectionedArray:(nonnull NSArray *)sectionedArray delegate:(nullable id<NITableViewModelDelegate>)delegate;
 
 #pragma mark Accessing Objects