Fixed bundle generate for not yet deployed apps (#3319)

## Changes
Fixed bundle generate for not yet deployed apps

## Why
It's common to run these commands next to each other
```
databricks apps create my-app
databricks bundle generate app --existing-app-name my-app
```

Due to the bug, we could not generate the app correctly since the app is
not yet deployed, and the source code path is empty, resulting in the
`Error: Path () doesn't start with '/'` error

## Tests
Added acceptance test

<!-- If your PR needs to be included in the release notes for next
release,
add a separate entry in NEXT_CHANGELOG.md as part of your PR. -->

Author: andrewnester

NEXT_CHANGELOG.md

@@ -11,6 +11,7 @@
 
 ### Bundles
 * Modified run flag validation to allow `--refresh` and `--full-refresh` flags to be used together ([#3300](https://github.com/databricks/cli/pull/3300))
+* Fixed bundle generate for not yet deployed apps ([#3319](https://github.com/databricks/cli/pull/3319))
 * Respect export format for notebooks on bundle generate ([#3316](https://github.com/databricks/cli/pull/3316))
 
 ### API Changes

acceptance/bundle/generate/app_not_yet_deployed/databricks.yml

@@ -0,0 +1,2 @@
+bundle:
+  name: "app_not_yet_deployed"

acceptance/bundle/generate/app_not_yet_deployed/out.app.yml

@@ -0,0 +1,6 @@
+resources:
+  apps:
+    out:
+      name: my-app
+      description: ""
+      source_code_path: src/app

acceptance/bundle/generate/app_not_yet_deployed/out.test.toml

@@ -0,0 +1,5 @@
+Local = true
+Cloud = false
+
+[EnvMatrix]
+  DATABRICKS_CLI_DEPLOYMENT = ["terraform", "direct-exp"]

acceptance/bundle/generate/app_not_yet_deployed/output.txt

@@ -0,0 +1,19 @@
+
+>>> [CLI] apps create my-app
+{
+  "app_status": {
+    "message":"Application is running.",
+    "state":"RUNNING"
+  },
+  "compute_status": {
+    "message":"App compute is active.",
+    "state":"ACTIVE"
+  },
+  "id":"1000",
+  "name":"my-app",
+  "url":"my-app-123.cloud.databricksapps.com"
+}
+
+>>> [CLI] bundle generate app --existing-app-name my-app --config-dir . --key out
+Loading app 'my-app' configuration
+App configuration successfully saved to out.app.yml

acceptance/bundle/generate/app_not_yet_deployed/script

@@ -0,0 +1,2 @@
+trace $CLI apps create my-app
+trace $CLI bundle generate app --existing-app-name my-app --config-dir . --key out

acceptance/bundle/generate/app_not_yet_deployed/test.toml

@@ -0,0 +1,2 @@
+Local = true
+Cloud = false

cmd/bundle/generate/app.go

@@ -52,9 +52,13 @@ func NewGenerateAppCommand() *cobra.Command {
 		downloader := generate.NewDownloader(w, sourceDir, configDir)
 
 		sourceCodePath := app.DefaultSourceCodePath
-		err = downloader.MarkDirectoryForDownload(ctx, &sourceCodePath)
-		if err != nil {
-			return err
+		// If the source code path is not set, we don't need to download anything.
+		// This is the case for apps that are not yet deployed.
+		if sourceCodePath != "" {
+			err = downloader.MarkDirectoryForDownload(ctx, &sourceCodePath)
+			if err != nil {
+				return err
+			}
 		}
 
 		// Making sure the source code path is relative to the config directory.