T15064: Several arcanist PHP 8.1 compat issues on Windows

Summary:
Ran into `strlen`/`strpos` issues while using Arcanist on Windows:
- Running `arc version` when not in a project/repository directory blew up (may not be specific to Windows).
- Determining mime type of binary file in repository fails.

Refs T15064

Test Plan: Ran diff including a binary file. Ran `arc version` when not in a project/repository directory.

Reviewers: O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

Subscribers: avivey, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15064

Differential Revision: https://we.phorge.it/D25534

src/filesystem/Filesystem.php

@@ -275,7 +275,7 @@ final class Filesystem extends Phobject {
       $trap->destroy();
 
       if (!$ok) {
-        if (strlen($err)) {
+        if ($err !== null && strlen($err)) {
           throw new FilesystemException(
             $to,
             pht(
@@ -307,7 +307,7 @@ final class Filesystem extends Phobject {
    * @task   file
    */
   public static function remove($path) {
-    if (!strlen($path)) {
+    if ($path == null || !strlen($path)) {
       // Avoid removing PWD.
       throw new Exception(
         pht(
@@ -673,7 +673,7 @@ final class Filesystem extends Phobject {
     }
 
     // If we come back with an encoding, strip it off.
-    if (strpos($mime_type, ';') !== false) {
+    if ($mime_type !== null && strpos($mime_type, ';') !== false) {
       list($type, $encoding) = explode(';', $mime_type, 2);
       $mime_type = $type;
     }

src/workingcopyidentity/ArcanistWorkingCopyIdentity.php

@@ -289,7 +289,7 @@ final class ArcanistWorkingCopyIdentity extends Phobject {
   }
 
   public function readLocalArcConfig() {
-    if (strlen($this->localMetaDir)) {
+    if ($this->localMetaDir !== null && strlen($this->localMetaDir)) {
       $local_path = Filesystem::resolvePath('arc/config', $this->localMetaDir);
 
       $console = PhutilConsole::getConsole();