[Portal, Analytics, User Signup Methods] Missing empty count

## Background
Device: Google Chrome on Mac
Build: Staging

## Description
There is no empty count in User Signup Methods section, and the pie chart is moved up making the UI unaligned

## Steps
1. Login to Authgear portal
2. Choose any app
3. Go to Analytics
4. See no empty count for User Signup Methods section
5. See the pie chart is moved up

## Expected result
Suggest to show count for each method and align the pie chart of Signup Conversation and User Signup Method sections

## Screenshot

Open the Authgear portal and follow the Analytics steps to inspect the User Signup Methods and Signup Conversation sections. Confirm the missing empty count and vertical misalignment in the staging build, then verify that each method shows a count and both pie charts are aligned.

Written by the indexing model from the issue text.