[MOB-738] Fixed NPE when cached gas price is null

[carvalho2707]

What does this PR do?

This PR fixes a crash reported in sentry when the gasprice cached is null

Database changed?

No

Where should the reviewer start?

• [ ] GasSettingsRepository.kt

How should this be manually tested?

Flow on how to test this or QA Tickets related to this use-case: MOB-738

What are the relevant tickets? https://sentry.aptoide.com/sentry/walletapp/issues/146094/

Tickets related to this pull-request: MOB-738

Questions:

Does this add new dependencies which need to be added? (Eg. new libs, new keys, etc.)

Code Review Checklist

• [ ] Architecture
• [ ] Documentation on public interfaces
• [ ] Database changed?
• [ ] If yes - Database migration?
• [ ] Remove comments & unused code & forgotten testing Logs
• [ ] Codestyle
• [ ] Functional QA tests pass