GH-47380: [Python] Apply maps_as_pydicts to Nested MapScalar Values (#47454)

### Rationale for this change
Currently, the `maps_as_pydicts` parameter to `MapScalar.as_py` does not work on nested maps. See below:

```
import pyarrow as pa

t = pa.struct([pa.field("x", pa.map_(pa.string(), pa.map_(pa.string(), pa.int8())))])
v = {"x": {"a": {"1": 1}}}
s = pa.scalar(v, type=t)
print(s.as_py(maps_as_pydicts="strict"))

# {'x': {'a': [('1', 1)]}}
```

In this ^ case, I'd want to get the value: `{'x': {'a': {'1': 1}}}`, such that round trips would work as expected.

### What changes are included in this PR?
Begin to apply the `maps_as_pydicts` to nested values in map types as well, update relevant test.
 
### Are these changes tested?
Yes

### Are there any user-facing changes?
Yes, just a user-facing fix.

* GitHub Issue: #47380

Lead-authored-by: Johanna <[email protected]>
Co-authored-by: zzkv <[email protected]>
Co-authored-by: Johanna <[email protected]>
Signed-off-by: Raúl Cumplido <[email protected]>

Author: jo-migo

python/pyarrow/scalar.pxi

@@ -1173,7 +1173,10 @@ cdef class MapScalar(ListScalar, Mapping):
         if not maps_as_pydicts:
             return list(self)
         result_dict = {}
-        for key, value in self:
+        if self.values is None:
+            return result_dict
+
+        for key, value in zip(self.keys(), self.values.field(self.type.item_field.name)):
             if key in result_dict:
                 if maps_as_pydicts == "strict":
                     raise KeyError(
@@ -1183,7 +1186,7 @@ cdef class MapScalar(ListScalar, Mapping):
                 else:
                     warnings.warn(
                         f"Encountered key '{key}' which was already encountered.")
-            result_dict[key] = value
+            result_dict[key] = value.as_py(maps_as_pydicts=maps_as_pydicts)
         return result_dict
 
     def keys(self):

python/pyarrow/tests/test_scalars.py

@@ -956,7 +956,7 @@ def test_map_scalar_as_py_with_custom_field_name():
     ).as_py() == [("foo", "bar")]
 
 
-def test_nested_map_types_with_maps_as_pydicts():
+def test_map_types_with_maps_as_pydicts():
     ty = pa.struct([
         pa.field('x', pa.map_(pa.string(), pa.int8())),
         pa.field('y', pa.list_(pa.map_(pa.string(), pa.int8()))),
@@ -966,3 +966,32 @@ def test_nested_map_types_with_maps_as_pydicts():
     s = pa.scalar(v, type=ty)
 
     assert s.as_py(maps_as_pydicts="strict") == v
+
+
+def test_nested_map_types_with_maps_as_pydicts():
+    ty = pa.struct(
+        [
+            pa.field('x', pa.map_(pa.string(), pa.map_(pa.string(), pa.int8()))),
+            pa.field(
+                'y', pa.list_(pa.map_(pa.string(), pa.map_(pa.string(), pa.int8())))
+            ),
+        ]
+    )
+
+    v = {'x': {'a': {'1': 1}}, 'y': [{'b': {'2': 2}}, {'c': {'3': 3}}]}
+    s = pa.scalar(v, type=ty)
+
+    assert s.as_py(maps_as_pydicts="strict") == v
+
+
+def test_map_scalar_with_empty_values():
+    map_type = pa.struct(
+        [
+            pa.field('x', pa.map_(pa.string(), pa.string())),
+        ]
+    )
+
+    v = {'x': {}}
+    s = pa.scalar(v, type=map_type)
+
+    assert s.as_py(maps_as_pydicts="strict") == v