fix: set battery-mode-line-string value when updating battery status

[bormoge]

Hello.

This change is tangentially related to #792. Recently, I noticed that battery-mode-line-string was not being updated on the tab-bar, so I checked doom-modeline-update-battery-status and I saw that it doesn't set any value for battery-mode-line-string. This commit fixes that.
With the following code, you'll notice that the time will appear on the tab-bar, and with this commit, so will the battery percentage:

(display-time-mode +1)
(display-battery-mode +1)
(tab-bar-mode +1)
(add-to-list 'tab-bar-format 'tab-bar-format-align-right 'append)
(add-to-list 'tab-bar-format 'doom-modeline-tab-bar-format-global 'append)