This should do what you want:
<div class="comeBack_up" *ngIf="(previous_info | json) != ({} | json)">
or shorter
<div class="comeBack_up" *ngIf="(previous_info | json) != '{}'">
Each {}
creates a new instance and ====
comparison of different objects instances always results in false
. When they are convert to strings ===
results to true