Use the default for an empty UPLOAD_DIR_LIMIT_MB.

An empty or bad value gave a limit of zero, so previews kept no images.

tests/php/Unit/UploadTest.php

@@ -136,6 +136,18 @@ class UploadTest extends TestCase {
     $this->assertSame(2 + 2 + 5, upload_dir_size());
   }
 
+  public function testTheUploadDirLimitComesFromTheConfiguration(): void {
+    $this->setEnv('UPLOAD_DIR_LIMIT_MB', '2.5');
+    $this->assertSame((int)(2.5 * 1024 * 1024), upload_dir_limit_bytes());
+  }
+
+  public function testAnEmptyOrBadUploadDirLimitMeansTheDefault(): void {
+    foreach (array('', ' ', 'lots', '0', '-2', 'none') as $value) {
+      $this->setEnv('UPLOAD_DIR_LIMIT_MB', $value);
+      $this->assertSame(100 * 1024 * 1024, upload_dir_limit_bytes(), "'$value'");
+    }
+  }
+
   public function testStoreUploadPrunesAndDropsTheLastUploadFirst(): void {
     $last = $this->pending();
     $files = $this->others();