Allow command options to have nullable types and default values (#1444)

pkl-core/src/main/java/org/pkl/core/runtime/CommandSpecParser.java

@@ -513,12 +513,6 @@ public final class CommandSpecParser {
       var typeNode = resolved.getFirst();
       isNullable = resolved.getSecond();
       defaultValue = CommandSpecParser.this.getDefaultValue(prop, requireExplicitDefault);
-      if (isNullable && defaultValue != null) {
-        throw exceptionBuilder()
-            .evalError("commandOptionTypeNullableWithDefaultValue", prop.getName())
-            .withSourceSection(prop.getHeaderSection())
-            .build();
-      }
 
       resolve(prop, typeNode);
       return this;

pkl-core/src/main/resources/org/pkl/core/errorMessages.properties

@@ -1098,11 +1098,6 @@ Found both `@Flag` and `@Argument` annotations for options property `{0}`.\n\
 \n\
 Only one option type may be specified.
 
-commandOptionTypeNullableWithDefaultValue=\
-Unexpected option property `{0}` with nullable type and default value.\n\
-\n\
-Options with default values must not be nullable.
-
 commandOptionUnsupportedType=\
 Command option property `{0}` has unsupported {1}type `{2}`.