null values for size in php7

[garak]

After upgrading from php 5.5 to php 7, my thumbnails stopped working. This is related to this incompatible change made in 7.
My previous configuration for a thumbnail was like thumbnail: { size: [84, ], mode: outbound } and I had to change it to { size: [84, null], mode: outbound } (notice the explicit null).
I think you should do at least one of the following:

• do not allow empty values for size
• add a control to code
• add a warning in docs