fix(ui-ux): convert screen continue button

[chloezxyy]

What this PR does / why we need it:

Added a check to see if target token has been selected before enabling 'Continue' button

Which issue(s) does this PR fixes?:

Fixes #

DFC-389

Additional comments?:

Developer Checklist:

• [ ] Read your code changes at least once
• [ ] Tested on iOS/Android device (e.g, No crashes, library supported etc.)
• [ ] No console errors on web
• [ ] Tested on Light mode and Dark mode*
• [ ] Your UI implementation visually matched the rendered design*
• [ ] Unit tests*
• [ ] Added e2e tests*
• [ ] Added translations*

[github-actions[bot]]

Missing Translations Report

The following translations are missing for this pull request.

{
    "missingLanguageItems": {
        "zh-Hans": {
            "missingCount": 0,
            "labels": {},
            "totalCount": 2028,
            "allLabels": "{}"
        },
        "zh-Hant": {
            "missingCount": 0,
            "labels": {},
            "totalCount": 2028,
            "allLabels": "{}"
        },
        "fr": {
            "missingCount": 0,
            "labels": {},
            "totalCount": 2028,
            "allLabels": "{}"
        },
        "es": {
            "missingCount": 0,
            "labels": {},
            "totalCount": 2028,
            "allLabels": "{}"
        },
        "it": {
            "missingCount": 0,
            "labels": {},
            "totalCount": 2028,
            "allLabels": "{}"
        }
    },
    "totalMissingCount": 0
}

[github-actions[bot]]

Build preview for DeFiChain Wallet is ready!

Built with commit 437ebd9b233593c4df189ad58e24916fcffdf82c

https://expo.io/@defichain/wallet?release-channel=pr-preview-4118

[codecov-commenter]

Codecov Report

Merging #4118 (2140af3) into main (b320ad0) will decrease coverage by 9.90%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #4118      +/-   ##
==========================================
- Coverage   56.13%   46.23%   -9.90%     
==========================================
  Files         438      438              
  Lines       12312    12313       +1     
  Branches     4069     4070       +1     
==========================================
- Hits         6911     5693    -1218     
- Misses       5317     6548    +1231     
+ Partials       84       72      -12     

Files	Coverage Δ
...igator/screens/Portfolio/screens/ConvertScreen.tsx	0.00% <0.00%> (-50.47%)	:arrow_down:

... and 65 files with indirect coverage changes

:mega: We’re building smart automated test selection to slash your CI/CD build times. Learn more