Issue:
fix(cards): Return a 200 response indicating that a customer is none

## Type of Change
- [x] Bugfix
- [ ] New feature
- [ ] Enhancement
- [ ] Refactoring
- [ ] Dependency updates
- [ ] Documentation
- [ ] CI/CD

## Description
Bug Description
CustomerPaymentMethodsListResponse returns 404 when there is no customer tied to a payment

Proposed Behavior
Return a 200 response somehow indicating that a customer is not associated with the payment

Current Behavior
A 404 response with "Customer does not exist in our records" is being returned, although the customer_id is null


### Additional Changes

- [x] This PR modifies the API contract
- [ ] This PR modifies the database schema
- [ ] This PR modifies application configuration/environment variables

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## Motivation and Context
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## How did you test it?
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## Checklist
<!-- Put an `x` in the boxes that apply -->

- [x] I formatted the code `cargo +nightly fmt --all`
- [x] I addressed lints thrown by `cargo clippy`
- [x] I reviewed the submitted code
- [ ] I added unit tests for my changes where possible
- [ ] I added a [CHANGELOG](/CHANGELOG.md) entry if applicable

Fixes https://github.com/juspay/hyperswitch/issues/3767

Resolution:
<think>
PR Context:
- Title: fix(cards): Return a 200 response indicating that a customer is none
- Merged: 2024-02-23T10:31:26Z
- Files changed: 3 files
- Commits: 2 commits

Implementation approach:
## Type of Change
<!-- Put an `x` in the boxes that apply -->

- [x] Bugfix
- [ ] New feature
- [ ] Enhancement
- [ ] Refactoring
- [ ] Dependency updates
- [ ] Documentation
- [ ] CI/CD

## Description
Bug Description
CustomerPaymentMethodsListResponse returns 404 when there is no customer tied to a payment

Proposed Behavior
Return a 200 response indicating that a customer is not associated with the payment

Current Behavior
A 404 response with "Customer does not exist in ou
</think>

To implement this, you'll need to modify:


**Detailed Implementation:**


**Commit 1:** fix(cards): Return a 200 response indicating that a customer is none (#3796)
Files to modify:


**crates/api_models/src/payment_methods.rs**
  Modify existing code

**crates/router/src/core/payment_methods/cards.rs**
  Modify existing code