utils: uuid: make operator bool explicit

Following up on 69fcc053bb

To prevent unintentional implicit conversions
e.g. to a number.

Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
Message-Id: <20220216081623.830627-1-bhalevy@scylladb.com>

utils/UUID.hh

@@ -115,7 +115,7 @@ public:
         return !most_sig_bits && !least_sig_bits;
     }
 
-    operator bool() const noexcept {
+    explicit operator bool() const noexcept {
         return !is_null();
     }