fix: update() with in-place callback returning None no longer overwrites field

[deepakganesh78]

Fixes #163

Problem

Since commit 7987969 (Fix issue with lambda based updates), Fields._update_base assigns the return value of the callable back to data[field]:

python data[field] = val(data[field], data, field)

This broke callbacks that modify data[field] in-place and return None (the pre-7987969 pattern that issue #163 relies on), because None overwrites the value the callback just set.

Index._update_base had the opposite inconsistency — it called the callable without assigning the return, so return-value callbacks were silently ignored for array indices.

Root cause

Neither Fields._update_base nor Index._update_base handled both callback styles (in-place and return-value).

Fix

Both methods now capture the callable's return value and only assign it if it is not None. If the callable returns None (explicitly or implicitly), the in-place modification is preserved.

⚠️ Behavior change: intentionally setting None via a returning callback

This fix means a callback like lambda orig, data, field: None can no longer be used to deliberately set a field to None via its return value. No existing test or documented behavior relies on this pattern, and it is the only reasonable way to restore compatibility with the in-place callback contract that existed before 7987969 and that issue #163 depends on. Callers who need to set None can still do so inside the callback body: data[field] = None.

For Index._update_base, this is a net improvement — return-value callbacks (e.g. lambda orig, data, idx: orig + 1) now work for array indices, where previously the return value was silently discarded.

Reproduction

`python
from jsonpath_ng import parse

def lowercase_inplace(orig, data, field):
data[field] = data[field].lower()
# returns None

data = {'Data_cat': {'data_entry': [
{'value': 0, 'UPPERCASE': 'UPPERCASE_A'},
{'value': 2, 'UPPERCASE': 'UPPERCASE_B'},
]}}
parse('$..UPPERCASE').update(data, lowercase_inplace)

Before fix: UPPERCASE fields become None

After fix: UPPERCASE fields become lowercase strings

`

Validation

• 2 new regression tests (in-place and return-value callbacks with $..field doubledot notation)
• Full test suite: 355 tests passed, 0 failures
• The only existing lambda update test (lambda x, y, z: x + 1) exercises Fields (not Index) and continues to pass