Pixel-sizing only works after setting image size percentage

[mrwweb]

Describe the bug
This is related to #12276, #12277, and #12168 in various ways.

When inserting an image in 5.0-RC1, the pixel sizer doesn't work unless a percentage is set first.

The pixel sizer jumps from it's initial value to 1 the when hitting up (i.e. it's as if the value is 0 and 1 is added) and the size in pixels is not set either.

It appears that the sizing kicks in once both dimensions are set. As I tested this, I think I realized that the pixel dimension fields aren't actually set with values but instead have placeholders. I suspect that's the root cause of the issue. (Placeholders are always confusing!)

To Reproduce

1. Insert an image
2. Hit the "Up" control on the number field of either the width or height pixel size settings fields

Expected behavior
Image is proportionally scaled by 1px in that dimension. If the image is 300px wide, the image is now 301px wide.

Screenshots

Desktop (please complete the following information):
Win 10, Firefox 63.0.3, WP 5.0-RC1, no plugins, Twenty Nineteen