proc_rom: don't assert on big actionless switch.

See the test case.  PROC_ROM will consider this for evaluation, even
though -- without any actions -- lhs is empty (but still "uniform").
A zero-width memory is constructed, which later fails check with:

ERROR: Assert `width != 0' failed in kernel/mem.cc:518.

Ensure we don't proceed if there's nothing to encode.

tests/proc/proc_rom.ys

@@ -186,4 +186,27 @@ design -stash preopt
 equiv_opt -assert -run prepare: dummy
 
 
+design -reset
 
+read_ilang <<EOT
+
+module \m
+  wire width 3 input 1 \a
+
+  process \p
+    switch \a
+      case 3'000
+      case 3'001
+      case 3'010
+      case 3'011
+      case 3'100
+      case 3'101
+      case 3'110
+      case 3'111
+    end
+  end
+end
+
+EOT
+
+proc_rom