Fix PHP 8.1 "strlen(null)" exception for "/bin/bulk export --query --format" without parameter

Summary:
`strlen()` was used in Phabricator to check if a generic value is a non-empty string.
This behavior is deprecated since PHP 8.1. Phorge adopts `phutil_nonempty_string()` as a replacement.

Note: this may highlight other absurd input values that might be worth correcting
instead of just ignoring. If phutil_nonempty_string() throws an exception in your
instance, report it to Phorge to evaluate and fix that specific corner case.

This patch also fixes two similar strlen() occurrences in the same source file.

```
ERROR 8192: strlen(): Passing null to parameter #1 ($string) of type string is deprecated at [/var/www/html/phorge/phorge/src/applications/transactions/bulk/management/PhabricatorBulkManagementExportWorkflow.php:167]
```

Test Plan: Run `../phorge/bin/bulk export --query --format`: Get no `strlen()` error anymore but only expected output `Usage Exception: Query "--format" is unknown. To run a builtin query like "all" or "active", also specify the search engine with "--class".`

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25588

src/applications/transactions/bulk/management/PhabricatorBulkManagementExportWorkflow.php

@@ -51,7 +51,7 @@ final class PhabricatorBulkManagementExportWorkflow
     list($engine, $queries) = $this->newQueries($args);
 
     $format_key = $args->getArg('format');
-    if (!strlen($format_key)) {
+    if (!phutil_nonempty_string($format_key)) {
       throw new PhutilArgumentUsageException(
         pht(
           'Specify an export format with "--format".'));
@@ -77,7 +77,7 @@ final class PhabricatorBulkManagementExportWorkflow
     $is_overwrite = $args->getArg('overwrite');
     $output_path = $args->getArg('output');
 
-    if (!strlen($output_path)) {
+    if (!phutil_nonempty_string($output_path)) {
       throw new PhutilArgumentUsageException(
         pht(
           'Use "--output <path>" to specify an output file, or "--output -" '.
@@ -164,7 +164,7 @@ final class PhabricatorBulkManagementExportWorkflow
       ->execute();
 
     $class = $args->getArg('class');
-    if (strlen($class)) {
+    if (phutil_nonempty_string($class)) {
 
       $class_list = array();
       foreach ($engine_classes as $class_name => $engine_object) {