Mark empty slots with invalid msgid, and don't try to unsubscribe all on them

[skliper]

Is your feature request related to a problem? Please describe.
Unsubscribe all currently loops through the entire table unsubscribing, including the MSGID 0x0's (which is a valid MSGID). This causes numerous errors:

EVS Port1 66/1/CFE_SB 12: Unsubscribe Err:No subs for Msg 0x0 on TO_LAB_TLM_PIPE,app TO_LAB_APP

Describe the solution you'd like
Mark as invalid, skip invalid

Describe alternatives you've considered
Keep a count

Additional context
Observed in fixing #71

Requester Info
Jacob Hageman - NASA/GSFC

[thnkslprpt]

@skliper could we just change the if condition before the attempt to unsubscribe:

to_lab/fsw/src/to_lab_cmds.c

Line 205 in 9b1d6d0

if (CFE_SB_IsValidMsgId(SubEntry->Stream))

to something like this?

if ((CFE_SB_IsValidMsgId(SubEntry->Stream)) && (CFE_SB_MsgIdToValue(SubEntry->Stream) != 0))

[skliper]

This issue is likely obsolete, since CFE_SB_INVALID_MSG_ID is now 0:
https://github.com/nasa/cFE/blob/7f5ebcd15032fe31e7ef0e5ff4ef624d51d66e16/modules/core_api/fsw/inc/cfe_sb_api_typedefs.h#L113

I suggest sending the to_lab remove all packets command and see if it now behaves as expected (shouldn't report errors on unsubscribing from 0x0).