Skip _oem_props if oem_dicts is None.

The oem_dicts in BuildInfo is only available to ota_from_target_files
when it's called with "--oem_settings" input. However, aosp/1135332
starts to use BuildInfo in add_img_to_target_files w/o oem_dicts. An
assert is triggered due to it when oem_fingerprint_properties is in the
info_dict.
This change skip _oem_props reading if oem_dicts is None to allow to
this case.

BUG: 155360923
BUG: 154171021
Test: Sign images with oem_fingerprint_properties.
Change-Id: I6a73feecd9567fd4d85f4eab3d4e11c5df28fe39