Spinner: Pressing increment/decrement button does nothing when value is blank

[clown]

Somewhat odd behaviour: If the text box component of a Spinner is blank, clicking and pressing the increment or decrement buttons do nothing, but they do appear pushed down.

If activated via the keyboard (up and down arrow keys), again the arrow buttons appear depressed, but the value of the spinner does not change -- the text box remains blank.

Suggestion: if blank and decrementing, show the spinner's maximum, and decrement it; if incrementing, show the spinner's minimum and increment it.

[bill]

From the meeting, we were worried about when min and/or max isn't specified... Doug suggested that if no min, use max and vis-a-versa, and if neither, use 0.

[doughays]

(In [13729]) Fixes #6398. If the value is not-a-number and the user requests spinning, then assign defaults based on min/max/0.