Revert "[adb] fix signature check"

This reverts commit c935d0c450.

Reason for revert: invalid test, let's change it instead of breaking incremental installations

Change-Id: Ie5fb318c26d3a74cd2dbb16e09bda018efece211

adb/client/incremental.cpp

@@ -122,7 +122,7 @@ static unique_fd start_install(const Files& files, const Args& passthrough_args,
         }
 
         auto [signature_fd, signature] = read_and_encode_signature(st.st_size, file, silent);
-        if (!signature_fd.ok() || signature.empty()) {
+        if (signature_fd.ok() && signature.empty()) {
             return {};
         }