fix!: support empty or default fields when updating templates (#19256)

Breaking change: Field types in `codersdk.UpdateTemplateMeta` for
`Icon`, `Description`, and `DisplayName` moved to `*string`

## Summary

In this pull request we're updating the `UpdateTemplateMeta` struct to
allow `DisplayName`, `Description`, and `Icon` to be set as empty `""`
or default to the value from the template if not provided in an update
call.

Fixes https://github.com/coder/coder/issues/19036

### The bug

The reported bug occurred when clients were attempting to update a
metadata field in a template via an edit call. When the request was
decoded into an `UpdateTemplateMeta` struct the default values for
fields in the struct were used to update the template even if they
weren't provided. This led to fields like `Icon` being set to `""` (the
default value).

### Changes

To allow for specific fields to be set to `""` these fields were updated
to be `*string` as opposed to `string`. This allows for clients to set
these fields as `""` in an update request or they will default to the
template value if they are not provided in the update request (will be
`nil`).

Added tests to confirm empty and nil values and updated other tests that
use these fields.

coderd/templates.go

@@ -771,12 +771,16 @@ func (api *API) patchTemplateMeta(rw http.ResponseWriter, r *http.Request) {
 		classicTemplateFlow = *req.UseClassicParameterFlow
 	}
 
+	displayName := ptr.NilToDefault(req.DisplayName, template.DisplayName)
+	description := ptr.NilToDefault(req.Description, template.Description)
+	icon := ptr.NilToDefault(req.Icon, template.Icon)
+
 	var updated database.Template
 	err = api.Database.InTx(func(tx database.Store) error {
 		if req.Name == template.Name &&
-			req.Description == template.Description &&
-			req.DisplayName == template.DisplayName &&
-			req.Icon == template.Icon &&
+			description == template.Description &&
+			displayName == template.DisplayName &&
+			icon == template.Icon &&
 			req.AllowUserAutostart == template.AllowUserAutostart &&
 			req.AllowUserAutostop == template.AllowUserAutostop &&
 			req.AllowUserCancelWorkspaceJobs == template.AllowUserCancelWorkspaceJobs &&
@@ -827,9 +831,9 @@ func (api *API) patchTemplateMeta(rw http.ResponseWriter, r *http.Request) {
 			ID:                           template.ID,
 			UpdatedAt:                    dbtime.Now(),
 			Name:                         name,
-			DisplayName:                  req.DisplayName,
-			Description:                  req.Description,
-			Icon:                         req.Icon,
+			DisplayName:                  displayName,
+			Description:                  description,
+			Icon:                         icon,
 			AllowUserCancelWorkspaceJobs: req.AllowUserCancelWorkspaceJobs,
 			GroupACL:                     groupACL,
 			MaxPortSharingLevel:          maxPortShareLevel,