Remove invalid comparison of va_list and nullptr (facebook#5836)

Summary:
The comparison of va_list and nullptr is always False under any arch, and will raise invalid operands of types error in aarch64 env (`error: invalid operands of types ‘va_list {aka __va_list}’ and ‘std::nullptr_t’ to binary ‘operator!=’`).

This patch removes this invalid assert.

Closes: facebook#4277
Pull Request resolved: facebook#5836

Differential Revision: D17532470

fbshipit-source-id: ca98078ecbc6a9416c69de3bd6ffcfa33a0f0185

java/rocksjni/loggerjnicallback.cc

@@ -131,7 +131,6 @@ void LoggerJniCallback::Logv(const InfoLogLevel log_level, const char* format,
     }
 
     assert(format != nullptr);
-    assert(ap != nullptr);
     const std::unique_ptr<char[]> msg = format_str(format, ap);
 
     // pass msg to java callback handler