Add early return to ToolBar::handleDPIChange in case it has no items

[fedejeanne]

If the toolbar has no items then the method still executes and sends 2 unnecessary OS calls:

• OS.SendMessage(toolBar.handle, OS.TB_BUTTONSTRUCTSIZE, TBBUTTON.sizeof, 0);
• OS.SendMessage(toolBar.handle, OS.TB_AUTOSIZE, 0, 0);

How to test

• Modify the Snippet18 and remove all items in the toolbar
• Run the modified Snippet18 with the vm arguments: -Dswt.autoScale.updateOnRuntime=true

Expected results

• Same visual result as the code in master
• The OS calls should not happen.

[github-actions]

Test Results

   486 files  ±0     486 suites  ±0   7m 52s ⏱️ +21s
 4 151 tests ±0   4 143 ✅ ±0   8 💤 ±0  0 ❌ ±0 
16 358 runs  ±0  16 266 ✅ ±0  92 💤 ±0  0 ❌ ±0 

Results for commit 07ae825. ± Comparison against base commit aed1b96.

♻️ This comment has been updated with latest results.

[HeikoKlare]

Change looks sound to me. Would be good to also have your opinion, @akoch-yatta.

[akoch-yatta]

Change looks sound to me. Would be good to also have your opinion, @akoch-yatta.

I just had one scenario in my mind that could be a problem, I need to try that out.

@HeikoKlare I wanted to play around with scaling empty toolbar and extending that afterwards, but that works fine as well. So no objection from my side