[flang][openacc] Fix wait clause printer (#137263)

wait clause printer is failing with case like: 

```
!$acc serial device_type(nvidia) wait
!$acc end serial
```

Author: clementval

flang/test/Lower/OpenACC/acc-serial.f90

@@ -87,6 +87,11 @@ subroutine acc_serial
 ! CHECK:        acc.yield
 ! CHECK-NEXT: }
 
+  !$acc serial device_type(nvidia) wait
+  !$acc end serial
+
+! CHECK: acc.serial wait([#acc.device_type<nvidia>])
+
   !$acc serial wait(1)
   !$acc end serial
 

mlir/lib/Dialect/OpenACC/IR/OpenACC.cpp

@@ -1411,20 +1411,22 @@ static void printWaitClause(mlir::OpAsmPrinter &p, mlir::Operation *op,
   if (hasDeviceTypeValues(keywordOnly) && hasDeviceTypeValues(deviceTypes))
     p << ", ";
 
-  unsigned opIdx = 0;
-  llvm::interleaveComma(llvm::enumerate(*deviceTypes), p, [&](auto it) {
-    p << "{";
-    auto boolAttr = mlir::dyn_cast<mlir::BoolAttr>((*hasDevNum)[it.index()]);
-    if (boolAttr && boolAttr.getValue())
-      p << "devnum: ";
-    llvm::interleaveComma(
-        llvm::seq<int32_t>(0, (*segments)[it.index()]), p, [&](auto it) {
-          p << operands[opIdx] << " : " << operands[opIdx].getType();
-          ++opIdx;
-        });
-    p << "}";
-    printSingleDeviceType(p, it.value());
-  });
+  if (hasDeviceTypeValues(deviceTypes)) {
+    unsigned opIdx = 0;
+    llvm::interleaveComma(llvm::enumerate(*deviceTypes), p, [&](auto it) {
+      p << "{";
+      auto boolAttr = mlir::dyn_cast<mlir::BoolAttr>((*hasDevNum)[it.index()]);
+      if (boolAttr && boolAttr.getValue())
+        p << "devnum: ";
+      llvm::interleaveComma(
+          llvm::seq<int32_t>(0, (*segments)[it.index()]), p, [&](auto it) {
+            p << operands[opIdx] << " : " << operands[opIdx].getType();
+            ++opIdx;
+          });
+      p << "}";
+      printSingleDeviceType(p, it.value());
+    });
+  }
 
   p << ")";
 }