Fix crash when dragging widget into empty hub

Use the calculated size calculated from the grid instead of checking
the width of the first visible item, which doesn't exist when the grid
is empty.

Bug: 416344560
Test: manually verified on device
Flag: com.android.systemui.glanceable_hub_v2
Change-Id: I2abde471214d122d3036c71fb2d3111dc9e2363c