Fix nil pointer dereference in Connect

[rembrandtreyes]

Description

This PR fixes a nil pointer dereference panic in recoverLoop that occurs when Connect() is called with a nil onConnect callback and a Redis reconnection event is triggered.

The fix normalizes a nil onConnect parameter to a no-op function at the API boundary in Connect(), preventing the panic in recoverLoop when it attempts to invoke the callback.

Changes:

• storage/connection_handler.go: Add nil check in Connect() to normalize nil callback to func() {}
• storage/connection_handler_test.go: Add two regression tests:
• TestConnectWithNilOnConnect: Verifies recoverLoop processes reconnect signals without panic
• TestConnectNormalizesNilCallback: Verifies Connect() with nil callback doesn't panic on reconnect

Related Issue

#7355

Motivation and Context

Several code paths call Connect() with a nil callback because they don't need reconnection notifications:

• gateway/coprocess_api.go (production code)
• Various test files

When Redis experiences a failover (e.g., Sentinel switching masters), statusCheck() detects the reconnection and sends a signal to recoverLoop(), which then attempts to call the nil callback, causing a panic:

panic: runtime error: invalid memory address or nil pointer dereference
goroutine 216 [running]:
github.com/TykTechnologies/tyk/storage.(*ConnectionHandler).recoverLoop(...)
    github.com/TykTechnologies/tyk/storage/connection_handler.go:122 +0x33

This fix ensures the API handles nil gracefully, which is the expected behavior for optional callbacks in Go.

How This Has Been Tested

1. Unit tests: Added two new tests that verify nil callback handling
2. Existing tests: All storage package tests pass (100+ tests)
3. Linting: golangci-lint passes with 0 issues

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Refactoring or add test (improvements in base code or adds test coverage to functionality)

Checklist

• I ensured that the documentation is up to date
• I explained why this PR updates go.mod in detail with reasoning why it's required
• I would like a code coverage CI quality gate exception and have explained why