fix: prevent closePosition panic when withdrawals map is empty

[liobrasil]

Summary

This PR fixes an edge-case panic in closePosition when the position has no debt and no remaining collateral at close time.

Real panic scenario (before this fix)

1. User opens a position and deposits 100.0 FLOW.
2. User never borrows.
3. User withdraws the full 100.0 FLOW.
4. User calls repay_and_close_position.cdc to close.

At this point in closePosition:

• debtsByType = {} (no debt to repay),
• collateralTypes = [],
• _withdrawAllCollateral(...) returns vaults = [].

The old code then built withdrawalsByType with:

• for i in InclusiveRange(0, vaults.length - 1)

With vaults.length == 0, this range evaluates to InclusiveRange(0, -1) (which iterates 0, -1).
The first iteration tries vaults[0] on an empty array, causing an out-of-bounds runtime panic and reverting close.

Root cause

Using an index-based loop that assumes a non-empty array when building withdrawalsByType.

Fix

In FlowALPv0.cdc close flow, replaced the inclusive-range index loop with a bounds-safe loop:

• var i = 0
• while i < vaults.length { ...; i = i + 1 }

This preserves behavior for non-empty arrays and safely no-ops for empty arrays.

Why this is safe

• No change to close business logic.
• No change to debt repayment semantics.
• No API changes.
• Change is localized to event-map construction (withdrawalsByType) and only affects iteration safety.

Tests

Added regression test:

• cadence/tests/close_position_empty_position_test.cdc

The test executes the real scenario above and asserts close succeeds without panic and funds are returned as expected.

Also re-ran existing close-related tests to ensure no regressions:

• close_position_dust_return_test.cdc
• close_position_precision_test.cdc
• close_position_queued_overpayment_test.cdc
• close_position_rounding_overpayment_test.cdc

Risk / Compatibility

• Low risk.
• Backward compatible.
• Prevents a production-blocking close failure for valid no-debt / fully-withdrawn positions.