Fix nil pointer dereference in config filtering (#3553)

Signed-off-by: Toni Kangas <toni.kangas@upcloud.com>

internal/configs/configschema/filter.go

@@ -57,7 +57,7 @@ func (b *Block) Filter(filterAttribute FilterT[*Attribute], filterBlock FilterT[
 			ret.Attributes[name] = &attr
 		}
 
-		if attr.NestedType != nil {
+		if ret.Attributes[name] != nil && attr.NestedType != nil {
 			ret.Attributes[name].NestedType = filterNestedType((&attr).NestedType, filterAttribute)
 		}
 	}

internal/configs/configschema/filter_test.go

@@ -176,6 +176,18 @@ func TestFilter(t *testing.T) {
 							Nesting: NestingList,
 						},
 					},
+					"read_only_nested": {
+						NestedType: &Object{
+							Attributes: map[string]*Attribute{
+								"string": {
+									Type:     cty.String,
+									Optional: true,
+								},
+							},
+							Nesting: NestingList,
+						},
+						Computed: true,
+					},
 				},
 
 				BlockTypes: map[string]*NestedBlock{