luci-base: addressing this.vstack[0] is undefined.

[adelton]

• This PR is not from my main or master branch 💩, but a separate branch ✅
• Each commit has a valid ✒️ Signed-off-by: <[email protected]> row (via git commit --signoff)
• Each commit and PR title has a valid 📝 <package name>: title first line subject for packages
• Incremented 🆙 any PKG_VERSION in the Makefile
• Tested on: x86_64 VM with OpenWrt 24.10.0-rc5 (r28304-6dacba30a7)
• ( Preferred ) Mention: @ the original code author for feedback
• ( Preferred ) Screenshot or mp4 of changes:
• ( Optional ) Closes: e.g. openwrt/luci#issue-number
• ( Optional ) Depends on: e.g. openwrt/packages#pr-number in sister repo
• Description: with a67d683, any attempt to Edit a network interface produces a yellow message TypeError: this.vstack[0] is undefined:
  
  I believe the reason is that .datatype cannot be an empty string. With this change the TypeError is gone.

[systemcrash]

Good catch. That does look better. Thanks @adelton