[BUG] If the result of a command is a single newline, assigning it to a variable results in an empty string

**To Reproduce**
A short version: `amber eval 'let source = "\n"; source = trust $ echo "\$\{{nameof source}}" $; trust $ echo \$\{{nameof source}@Q} $'` → `''` (`$'\n'` is expected)

Longer:

```js
fun print_h(text) {
trust $ echo \$\{{nameof text}@Q} $
}

let source = "\n"
// original
print_h(source)
// assign
source = trust $ echo "\$\{{nameof source}}" $
// should be the same as the original
print_h(source)
```

Unexpectedly, the source is replaced with an empty string.

```
$'\n'
''
```

**Expected behavior**
```
$'\n'
$'\n'
```

**Additional context**

1. Here is the transpiled result of a short version:
```console
echo 'let source = "\n"; source = trust $ echo "\$\{{nameof source}}" $; trust $ echo \$\{{nameof source}@Q} $' | amber build - -
#!/usr/bin/env bash
# Written in [Amber](https://amber-lang.com/)
# version: 0.4.0-alpha
# date: 2024-12-24 22:10:02
__0_source="
"
__AMBER_VAL_0=$(echo "${__0_source}")
__AS=$?
__0_source="${__AMBER_VAL_0}"
echo ${__0_source@Q}
__AS=$?
```

if `__AMBER_VAL_0=$(echo "${__0_source}")` is replaced with `__AMBER_VAL_0="$(echo \"${__0_source}\")"`, the problem disappears.

2. This is not related to `IFS`.