If you don't want to see that warning (I'm guessing you see it on the monitor), you can switch the order of the first condition for the Filter expression (see below). The Or operator can "short-circuit" the evaluation: if the first operand is true, then it will not evaluate the other operands, so in this case it will not call the Value function with an empty value.
Search(
Filter(
SearchDateRange,
FormID = IsBlank(txtQAFormIDSearch.Text) Or Value(txtQAFormIDSearch.Text),
AgentPersonNumber = cboAgentSearch.Selected.PersonNumber Or IsBlank(cboAgentSearch.Selected.PersonNumber),
QualityCoachName = cboQACoachSearch.Selected.QualityCoachName Or IsBlank(cboQACoachSearch.Selected.QualityCoachName)
),
txtPolicyReferenceSearch.Text,
"CallReference",
"PolicyReference"
)
Hope this helps!