Fix NPE in getCompatibleMagnificationSpecForWindow

The commit 179a6b06 forgot to change the returned result.

Note that if the "spec" is non-null, the old code will
cause the source spec of DisplayMagnifier to be cumulatively
multiplied and then become infinite scale.

Bug: 180050756
Test: Enable compat scale and TalkBack
Change-Id: I5a647d4129165021054631461fb97aef61def390