[DocsUI M3] Fix crash in grid mode due to missing method in older JDK

Removed usage of CharSequence#isEmpty. This method is not available
in builds made using JDK older than 15

Bug: 403484805
Test: m DocumentsUIGoogle && manual inspection
Flag: com.android.documentsui.flags.use_material3
Change-Id: Id5a8792b012fe079e88315d0bb85527f5b76bc6b