RLCMAC: Fix decoding of dl data block with no llc block header byte

The value was marked as unbound while it should be marked as omitted:

"""
dl_block := { data := { cs := CS_1 (0), mac_hdr := { mac_hdr := { payload_type := MAC_PT_RLC_DATA (0), rrbp := RRBP_Nplus13_mod_2715648 (0), rrbp_valid := false, usf := 7 }, hdr_ext := { pr := PWR_RED_0_to_3dB (0), spare := '0'B, tfi := 0, fbi := false, bsn := 0, e := true } }, blocks := { { hdr := <unbound>, payload := '0D0C8653110FFEF072972C5A790C0317299753B5'O } } } } }
"""

Change-Id: If00338729ee1b2c8a4e100495e7cfa0599a4cb8b
1 file changed