You'd use an iterator if you've already got the expressions to feed in...
In combination with that, you need logic Get Count, see this below - ironically in the very 1st picture it shows the very ModelBuilder logic you want to apply before a buffer execution:
Get Count (Data Management)
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From the model perspective, this can be illustrated very simply. This is a graphic of a working model employing the Get Count precondition technique:
PARCELS is a parcel polygon layer in my open ArcMap document with only 2 tools employed (for simplicity), Get Count and Buffer, similar to the example from the webhelp link above. The reason I simplified the example with this model is to demonstrate the interaction more directly with the PARCELS layer which I've applied a definition query to - then I made 2 model executions, one with the def qry purposely made to return nothing (count = 0) and the other to return 1 feature (which is then essentially a pass filter to go on to the next process, Buffer).
Notice the dotted line representing the 'connector' between Row Count and Buffer - this denotes the Precondition. One way to apply this precondition is to right-click on Buffer (in this case), select properties, click on the Preconditions tab, and select Row Count (toggle checkmark) for the Precondition Variable like so:
(Alternately, you could use the connector tool to connect Row Count to Buffer and in the pop-up select Precondition.)
As for the 2 demos, a def query set to return None (no features) and another to return 1 feature, this is the dialog on None (a false condition):
...and, of course, altering the def query to return something (1 feature, in this case), this is what I call the 'pass logic', allowing Buffer to execute (a true condition):