clang-format: allow some empty blocks on single line

Enable AllowShortBLocksOnASingleLine and
AllowShortFunctionsOnASingleLine as 'Empty' instead of 'false'.  This
allows the code to be slightly more compact in the already readable
case of an empty block.  In sdbusplus this yields some minor
simplifications such as:

```
-    compose(bus_t&, const char*)
-    {}
+    compose(bus_t&, const char*) {}
```

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I39423dc80c057674e16d23389f4e4d88aae80e6f
diff --git a/style/cpp/.clang-format b/style/cpp/.clang-format
index 00e2e25..ba9f48d 100644
--- a/style/cpp/.clang-format
+++ b/style/cpp/.clang-format
@@ -9,9 +9,9 @@
 AlignOperands:   true
 AlignTrailingComments: true
 AllowAllParametersOfDeclarationOnNextLine: true
-AllowShortBlocksOnASingleLine: false
+AllowShortBlocksOnASingleLine: Empty
 AllowShortCaseLabelsOnASingleLine: false
-AllowShortFunctionsOnASingleLine: None
+AllowShortFunctionsOnASingleLine: Empty
 AllowShortIfStatementsOnASingleLine: false
 AllowShortLoopsOnASingleLine: false
 AlwaysBreakAfterReturnType: None