Ensure SOONG_CONV_DATA is generated even no modules require conversion

This change ensures that the `SOONG_CONV_DATA` file is always
generated, even when there are no modules that require conversion from
Make to Soong.
Previously, if no Android.mk files were present in the source code, the
`SOONG_CONV` variable would be empty, preventing the creation of
`SOONG_CONV_DATA` and causing build failures.

Bug: 382385324
Test: m $ANDROID_PRODUCT_OUT/obj/PACKAGING/soong_conversion_intermediates/soong_conv_data
Change-Id: Ifd9ec50e1615765f0d440a767a35f87d31a251f0