fix: only set avtar img src attribute if property is defined

[web-padawan]

Description

Currently, in Lit version the empty src attribute is set by default (can be checked by snapshot test). This PR fixes that:

 ❌ vaadin-avatar > default
      AssertionError: Snapshot vaadin-avatar default does not match the saved snapshot on disk
      + expected - actual

       <img
         aria-hidden="true"
         draggable="false"
         hidden=""
      -  src=""
       >
       <slot name="tooltip">
       </slot>

Type of change

• Bugfix

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud