fix: Pubspec.environment can never be null

CHANGELOG.md

@@ -1,6 +1,7 @@
 ## 1.3.1-wip
 
 - Require Dart 3.2
+- Set `Pubspec.environment` to non-nullable.
 - Remove deprecated package_api_docs rule
 
 ## 1.3.0

lib/src/pubspec.dart

@@ -74,7 +74,7 @@ class Pubspec {
   final String? documentation;
 
   @JsonKey(fromJson: _environmentMap)
-  final Map<String, VersionConstraint?>? environment;
+  final Map<String, VersionConstraint?> environment;
 
   @JsonKey(fromJson: parseDeps)
   final Map<String, Dependency> dependencies;
@@ -186,7 +186,7 @@ class Pubspec {
 Version? _versionFromString(String? input) =>
     input == null ? null : Version.parse(input);
 
-Map<String, VersionConstraint?>? _environmentMap(Map? source) =>
+Map<String, VersionConstraint?> _environmentMap(Map? source) =>
     source?.map((k, value) {
       final key = k as String;
       if (key == 'dart') {
@@ -222,4 +222,5 @@ Map<String, VersionConstraint?>? _environmentMap(Map? source) =>
       }
 
       return MapEntry(key, constraint);
-    });
+    }) ??
+    {};