Fix crashes when mentioning users without display names

[stefanceriu]

Fixes #7579

[sonarcloud[bot]]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
No Duplication information

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (c87b8e2) 12.22% compared to head (743778f) 12.22%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #7581 +/- ## ======================================== Coverage 12.22% 12.22% ======================================== Files 1648 1648 Lines 163591 163590 -1 Branches 67156 67156 ======================================== Hits 19992 19992 + Misses 142954 142953 -1 Partials 645 645 ``` | Flag | Coverage Δ | | |---|---|---| | uitests | `55.05% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vector-im#carryforward-flags-in-the-pull-request-comment) to find out more. | [Impacted Files](https://app.codecov.io/gh/vector-im/element-ios/pull/7581?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vector-im) | Coverage Δ | | |---|---|---| | [Riot/Modules/Pills/PillTextAttachment.swift](https://app.codecov.io/gh/vector-im/element-ios/pull/7581?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vector-im#diff-UmlvdC9Nb2R1bGVzL1BpbGxzL1BpbGxUZXh0QXR0YWNobWVudC5zd2lmdA==) | `69.23% <ø> (+0.75%)` | :arrow_up: |

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