FHIRPath constant value null casting bug fix

[FikriMilano]

IMPORTANT: All PRs must be linked to an issue (except for extremely trivial and straightforward changes).

Fixes #2011

Description
Use safe casting

Alternative(s) considered
N/A

Type
Bug fix

Screenshots (if applicable)
N/A

Checklist

• I have read and acknowledged the Code of conduct.
• I have read the Contributing page.
• I have signed the Google Individual CLA, or I am covered by my company's Corporate CLA.
• I have discussed my proposed solution with code owners in the linked issue(s) and we have agreed upon the general approach.
• I have run ./gradlew spotlessApply and ./gradlew spotlessCheck to check my code follows the style guide of this project.
• I have run ./gradlew check and ./gradlew connectedCheck to test my changes locally.
• I have built and run the demo app(s) to verify my change fixes the issue and/or does not break the demo app(s).

[santosh-pingle]

thanks! @FikriMilano
LGTM,

[FikriMilano]

@santosh-pingle can you merge the PR as well? I don't have write access to do that

[santosh-pingle]

@santosh-pingle can you merge the PR as well? I don't have write access to do that

@jingtang10 Can you please review it