Add another line in the generate constant header script

The generated headers are currently missing a newline prior to @ifdef cpp_examples which is causing doc generation to fail.

PiperOrigin-RevId: 321272103

Author: a-maurice

analytics/generate_constants.py

@@ -91,6 +91,7 @@
     (r'(?s)(@code)([^{].*using namespace firebase::analytics;'
      r'.*Parameter [^{]+[^}]+}.*@endcode)',
      '\n'
+     '///\n'
      '/// @if cpp_examples\n'
      '/// \\1{.cpp}'
      '\\2\n\n'