Nullability warnings on Xcode 11

[Mark Lilback]

There are 27 warnings about a lack of Nullability specifiers when compiled with Xcode 11. Here is an example of one of them

In file included from /Users/mlilback/working/rc2/client/vendor/iosMath/iosMath/render/MTMathListDisplay.m:20:
/Users/mlilback/working/rc2/client/vendor/iosMath/iosMath/render/internal/MTMathListDisplayInternal.h:34:4: warning: pointer is missing a nullability type specifier (_Nonnull, _Nullable, or _Null_unspecified) [-Wnullability-completeness]
- (instancetype) initWithDisplays:(NSArray<MTDisplay*>*) displays range:(NSRange) range NS_DESIGNATED_INITIALIZER;
   ^
/Users/mlilback/working/rc2/client/vendor/iosMath/iosMath/render/internal/MTMathListDisplayInternal.h:34:4: note: insert '_Nullable' if the pointer may be null
- (instancetype) initWithDisplays:(NSArray<MTDisplay*>*) displays range:(NSRange) range NS_DESIGNATED_INITIALIZER;
   ^
                _Nullable 
/Users/mlilback/working/rc2/client/vendor/iosMath/iosMath/render/internal/MTMathListDisplayInternal.h:34:4: note: insert '_Nonnull' if the pointer should never be null
- (instancetype) initWithDisplays:(NSArray<MTDisplay*>*) displays range:(NSRange) range NS_DESIGNATED_INITIALIZER;
   ^
                _Nonnull 

I know that requirement was added years ago, but maybe the warning wasn't on by default. I'd submit a patch, but I'm not familiar enough to know what is nullable (except _Null_unspecified, which is the worst solution).