refactor: only substitute with non-empty rules

Author: AayushSabharwal

src/systems/connectors.jl

@@ -895,10 +895,12 @@ function expand_connections(sys::AbstractSystem; tol = 1e-10)
     stream_eqs, instream_subs = expand_instream(instream_csets, sys; tol = tol)
 
     eqs = [equations(sys); ceqs; stream_eqs]
-    # substitute `instream(..)` expressions with their new values
-    for i in eachindex(eqs)
-        eqs[i] = fixpoint_sub(
-            eqs[i], instream_subs; maxiters = max(length(instream_subs), 10))
+    if !isempty(instream_subs)
+        # substitute `instream(..)` expressions with their new values
+        for i in eachindex(eqs)
+            eqs[i] = fixpoint_sub(
+                eqs[i], instream_subs; maxiters = max(length(instream_subs), 10))
+        end
     end
     # get the defaults for domain networks
     d_defs = domain_defaults(sys, domain_csets)