allow empty dynamic blocks with ConfigModeAttr

If a dynamic block is evaluated zero times, the body content will
contain 0 blocks. Allow the probe for ConfigModeAttr to accept that no
blocks with a matching attribute should still be converted to a block if
they are called with dynamicExpand.

lang/blocktoattr/fixup_test.go

@@ -196,6 +196,20 @@ dynamic "foo" {
 				}),
 			}),
 		},
+		"dynamic block with empty iterator": {
+			src: `
+dynamic "foo" {
+  for_each = []
+  content {
+    bar = foo.value
+  }
+}
+`,
+			schema: fooSchema,
+			want: cty.ObjectVal(map[string]cty.Value{
+				"foo": cty.NullVal(fooSchema.Attributes["foo"].Type),
+			}),
+		},
 		"both attribute and block syntax": {
 			src: `
 foo = []

lang/blocktoattr/schema.go

@@ -55,10 +55,11 @@ func effectiveSchema(given *hcl.BodySchema, body hcl.Body, ambiguousNames map[st
 				},
 			}
 			content, _, _ = body.PartialContent(&probeSchema)
-			if len(content.Blocks) > 0 {
+			if len(content.Blocks) > 0 || dynamicExpanded {
-				// No attribute present and at least one block present, so
+				// A dynamic block with an empty iterator returns nothing.
-				// we'll need to rewrite this one as a block for a successful
+				// If there's no attribute and we have either a block or a
-				// result.
+				// dynamic expansion, we need to rewrite this one as a
+				// block for a successful result.
 				appearsAsBlock[name] = struct{}{}
 			}
 		}