Issue title: [iOS] [Teams] App Crashing on Rendering The Card with Images. Reproducible On sample App too.
 Issue body: Crash Log :- 
(lldb) po exception
```
Multiplier is not finite!  That's illegal.  multiplier:nan
(
	0   CoreFoundation                      0x000000011bf091e6 __exceptionPreprocess + 294
	1   libobjc.A.dylib                     0x000000011b5a6031 objc_exception_throw + 48
	2   CoreFoundation                      0x000000011bf0e472 +[NSException raise:format:arguments:] + 98
	3   Foundation                          0x00000001168ae652 -[NSAssertionHandler handleFailureInMethod:object:file:lineNumber:description:] + 193
	4   Foundation                          0x0000000116832840 -[NSLayoutConstraint _setMultiplier:] + 336
	5   Foundation                          0x0000000116832567 +[NSLayoutConstraint constraintWithItem:attribute:relatedBy:toItem:attribute:multiplier:constant:] + 248
	6   AdaptiveCards                       0x000000010ed16087 __36-[ACRView processImageConcurrently:]_block_invoke_2 + 421
	7   libdispatch.dylib                   0x000000011796c807 _dispatch_call_block_and_release + 12
	8   libdispatch.dylib                   0x000000011796d848 _dispatch_client_callout + 8
	9   libdispatch.dylib                   0x000000011797892b _dispatch_main_queue_callback_4CF + 628
	10  CoreFoundation                      0x000000011becbc99 __CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__ + 9
	11  CoreFoundation                      0x000000011be8fea6 __CFRunLoopRun + 2342
	12  CoreFoundation                      0x000000011be8f30b CFRunLoopRunSpecific + 635
	13  GraphicsServices                    0x000000011e68ca73 GSEventRunModal + 62
	14  UIKit                               0x0000000117ad30b7 UIApplicationMain + 159
	15  TeamsAppCore                        0x000000010f75d4aa AXPApplicationMain + 58
	16  TeamSpaceApp                        0x000000010e2c0ec9 main + 393
	17  libdyld.dylib                       0x000000011c402955 start + 1
)

```

JSON Payload 
{"body":[{"columns":[{"width":"2","items":[{"size":"medium","weight":"bolder","type":"TextBlock","text":"Tell us about yourself"},{"text":"We just need a few more details to get you booked for the trip of a lifetime!","isSubtle":true,"type":"TextBlock","wrap":true},{"size":"small","text":"Don't worry, we'll never share or sell your information.","isSubtle":true,"type":"TextBlock","wrap":true},{"type":"TextBlock","text":"Your name","wrap":true},{"placeholder":"Last, First","id":"myName","type":"Input.Text"},{"type":"TextBlock","text":"Your email","wrap":true},{"id":"myEmail","placeholder":"dustintodd@example.net","style":"email","type":"Input.Text"},{"type":"TextBlock","text":"Phone Number"},{"id":"myTel","placeholder":"xxx.xxx.xxxx","style":"tel","type":"Input.Text"}],"type":"Column"},{"width":"1","items":[{"url":"https:\/\/urlp.asm.skype.com\/v1\/url\/content?url=https%3a%2f%2fupload.wikimedia.org%2fwikipedia%2fcommons%2fb%2fb2%2fDiver_Silhouette%252C_Great_Barrier_Reef.jpg","type":"Image"}],"type":"Column"}],"type":"ColumnSet"}],"type":"AdaptiveCard","actions":[{"title":"Submit","type":"Action.Submit"}],"version":"1.0"}
 Comments: 
 Comment 0: dupe of issue #1418 
{'source': 'GithubIssues', 'repo_name_with_owner': 'Microsoft/AdaptiveCards', 'issue_id': 320228948, 'repo_stars': 465.0}