command/init: omit a warning if -backend-config is used with no backend (#22164)

* command/init: omit a warning if -backend-config is used with no backend
block

Terraform would silently accept - and swallow - `-backend-config` on the
CLI when there was no `backend` block. Since it is mostly expected to
override existing backend configuration, terraform
should omit a warning if there is no backend configuration to
override.

If the user intended to override the default (local) backend
configuration, they can first add a `backend` block to the `terraform` block to silence the warning (or just ignore it):

```hcl
terraform {
  backend "local" {}
}
```

command/init.go

@@ -438,6 +438,29 @@ func (c *InitCommand) initBackend(root *configs.Module, extraConfig rawFlags) (b
 		if overrideDiags.HasErrors() {
 			return nil, true, diags
 		}
+	} else {
+		// If the user supplied a -backend-config on the CLI but no backend
+		// block was found in the configuration, it's likely - but not
+		// necessarily - a mistake. Return a warning.
+		if !extraConfig.Empty() {
+			diags = diags.Append(tfdiags.Sourceless(
+				tfdiags.Warning,
+				"Missing backend configuration",
+				`-backend-config was used without a "backend" block in the configuration.
+
+If you intended to override the default local backend configuration,
+no action is required, but you may add an explicit backend block to your
+configuration to clear this warning:
+
+terraform {
+  backend "local" {}
+}
+
+However, if you intended to override a defined backend, please verify that
+the backend configuration is present and valid.
+`,
+			))
+		}
 	}
 
 	opts := &BackendOpts{

command/init_test.go

@@ -518,6 +518,32 @@ func TestInit_backendConfigKVReInitWithConfigDiff(t *testing.T) {
 	}
 }
 
+func TestInit_backendCli_no_config_block(t *testing.T) {
+	// Create a temporary working directory that is empty
+	td := tempDir(t)
+	copy.CopyDir(testFixturePath("init"), td)
+	defer os.RemoveAll(td)
+	defer testChdir(t, td)()
+
+	ui := new(cli.MockUi)
+	c := &InitCommand{
+		Meta: Meta{
+			testingOverrides: metaOverridesForProvider(testProvider()),
+			Ui:               ui,
+		},
+	}
+
+	args := []string{"-backend-config", "path=test"}
+	if code := c.Run(args); code != 0 {
+		t.Fatalf("got exit status %d; want 0\nstderr:\n%s\n\nstdout:\n%s", code, ui.ErrorWriter.String(), ui.OutputWriter.String())
+	}
+
+	errMsg := ui.ErrorWriter.String()
+	if !strings.Contains(errMsg, "Warning: Missing backend configuration") {
+		t.Fatal("expected missing backend block warning, got", errMsg)
+	}
+}
+
 func TestInit_targetSubdir(t *testing.T) {
 	// Create a temporary working directory that is empty
 	td := tempDir(t)