Workflow single-node execution drops explicit falsy inputs

## Dify version

main (`40df83de`)

## Cloud or Self Hosted

Self Hosted (Source)

## Steps to reproduce

1. Run a workflow node through the debugger or single-node execution path.
2. Map an input such as `node.enabled` to a downstream selector.
3. Provide `False`, `0`, or an empty string as the explicit value for that input.

## Expected Behavior

The explicit value is placed in the variable pool unchanged.

## Actual Behavior

The input is treated as absent. The mapper falls through to the short-key alias and, when that alias is absent, does not add the value to the variable pool.

## Root cause

`WorkflowEntry.mapping_user_inputs_to_variable_pool` uses a truthiness check before falling back from the full input key to its short-key alias. Valid falsy values therefore follow the missing-value path.

Start at WorkflowEntry.mapping_user_inputs_to_variable_pool, the entry point named in the report, and reproduce the debugger or single-node execution path with False, 0, and an empty string. Done means each explicit value reaches the variable pool unchanged instead of falling through to the short-key alias.

Written by the indexing model from the issue text.