Use `company-space-string' to generate whitespaces.

* company.el(company--create-line-overlay,
  company-pseudo-tooltip-unhide-line): Use `company-space-string'
  to generate whitespaces.
Fix issue of prefix not displaying when buffer line length is 0.

* company.el:

(company-pseudo-tooltip-hide): Call
`company-pseudo-tooltip-hide-line' on line overlays to restore
text properties before deleting overlay.

(company-pseudo-tooltip-unhide-line): If overlay length is 0, its
[line|wrap]-prefix property will have no effect, so set it on the
buffer text instead.

(company-pseudo-tooltip-hide-line): Restore text properties if
modified by `company-pseudo-tooltip-unhide-line'.