Fix null pointer passed to non-null argument in CHIPMemString.h

when PI= is in mDNS TXT record (its value is empty) , and Dnssd::Internal::GetPairingInstruction calls CopyString, source is an empty bytespan and source.data() will return a null pointer, that will be passed to memcpy
Fix: avoid memcpy in that case.

src/lib/support/CHIPMemString.h

@@ -82,12 +82,20 @@ inline void CopyString(char (&dest)[N], const char * source)
  */
 inline void CopyString(char * dest, size_t destLength, ByteSpan source)
 {
-    if (dest && destLength)
+    if ((dest == nullptr) || (destLength == 0))
     {
-        size_t maxChars = std::min(destLength - 1, source.size());
-        memcpy(dest, source.data(), maxChars);
-        dest[maxChars] = '\0';
+        return; // no space to copy anything, not even a null terminator
     }
+
+    if (source.empty())
+    {
+        *dest = '\0'; // just a null terminator, we are copying empty data
+        return;
+    }
+
+    size_t maxChars = std::min(destLength - 1, source.size());
+    memcpy(dest, source.data(), maxChars);
+    dest[maxChars] = '\0';
 }
 
 /**