Printing a Throwable throws if Throwable has no cause / stacktrace

Throwable->map in core_print.clj doesn't handle Throwable.getCause returning null in L463. This results in a NPE in StrackTraceElement->vec in the same file in some cases, so printing a stacktrace results in a new exception being thrown which is a bit confusing.

Repro:

Approach: Check that at least one StackTraceElement exists before using the top frame. Make printing tolerant of a missing :at value. Add test for this omitted stack trace case.

Patch: clj-1977.patch