Fix case when using multiple platforms but pip contains a single plat…

…form

Prevents
```
  File "/Users/bas.nijholt/micromamba/lib/python3.11/site-packages/conda_lock/src_parser/environment_yaml.py", line 58, in _parse_environment_file_for_platform
    for spec in mapping_spec["pip"]:
TypeError: 'NoneType' object is not iterable
```
Which occurs when locking:
```
name: example
channels:
  - conda-forge
dependencies:
  - tomli
  - pip:
    - psutil  # [linux64]
platforms:
  - linux-64
  - osx-arm64
```
Which becomes:
```
env_yaml_data = {'name': 'test-pip-with-platform-selector', 'channels': ['conda-forge'], 'dependencies': ['tomli', {'pip': None}], 'platforms': ['linux-64', 'osx-arm64']}
```
For `osx-arm64`.

This fix skips `{"pip": None}`.

conda_lock/src_parser/environment_yaml.py

@@ -54,29 +54,32 @@ def _parse_environment_file_for_platform(
         dependencies.append(conda_spec_to_versioned_dep(spec, category))
 
     for mapping_spec in mapping_specs:
-        if "pip" in mapping_spec:
-            for spec in mapping_spec["pip"]:
-                if re.match(r"^-e .*$", spec):
-                    print(
-                        (
-                            f"Warning: editable pip dep '{spec}' will not be included in the lock file. "
-                            "You will need to install it separately."
-                        ),
-                        file=sys.stderr,
-                    )
-                    continue
-
-                dependencies.append(
-                    parse_python_requirement(
-                        spec,
-                        manager="pip",
-                        category=category,
-                        normalize_name=False,
-                    )
+        pip = mapping_spec.get("pip")
+        if pip is None:
+            # might not be present OR might be None due to platform selector
+            continue
+        for spec in pip:
+            if re.match(r"^-e .*$", spec):
+                print(
+                    (
+                        f"Warning: editable pip dep '{spec}' will not be included in the lock file. "
+                        "You will need to install it separately."
+                    ),
+                    file=sys.stderr,
                 )
+                continue
+
+            dependencies.append(
+                parse_python_requirement(
+                    spec,
+                    manager="pip",
+                    category=category,
+                    normalize_name=False,
+                )
+            )
 
-            # ensure pip is in target env
-            dependencies.append(parse_python_requirement("pip", manager="conda"))
+        # ensure pip is in target env
+        dependencies.append(parse_python_requirement("pip", manager="conda"))
 
     return dependencies
 

tests/test-pip-with-platform-selector/environment.yml

@@ -0,0 +1,10 @@
+name: test-pip-with-platform-selector
+channels:
+  - conda-forge
+dependencies:
+  - tomli
+  - pip:
+    - psutil  # [linux64]
+platforms:
+  - linux-64
+  - osx-arm64