return empty list when encoding http header of undefined ctx (#124)

Author: tsloughter

src/oc_span_ctx_header.erl

@@ -43,7 +43,9 @@ encode(#span_ctx{trace_id=TraceId,
     Options = case TraceOptions band 1 of 1 -> <<"01">>; _ -> <<"00">> end,
     EncodedTraceId = io_lib:format("~32.16.0b", [TraceId]),
     EncodedSpanId = io_lib:format("~16.16.0b", [SpanId]),
-    [?VERSION, "-", EncodedTraceId, "-", EncodedSpanId, "-", Options].
+    [?VERSION, "-", EncodedTraceId, "-", EncodedSpanId, "-", Options];
+encode(undefined) ->
+    [].
 
 -spec decode(iodata()) -> maybe(opencensus:span_ctx()).
 decode(TraceContext) when is_list(TraceContext) ->

test/oc_span_ctx_SUITE.erl

@@ -89,6 +89,9 @@ encode_decode_headers(_Config) ->
     InvalidBothIdsHeader = <<"00-00000000000000000000000000000000-0000000000000000-00">>,
     undefined = oc_span_ctx_header:decode(InvalidBothIdsHeader),
 
+    NoCtx = undefined,
+    NoCtxEncoded = oc_span_ctx_header:encode(NoCtx),
+    ?assertEqual(NoCtx, oc_span_ctx_header:decode(NoCtxEncoded)),
 
     %% Encode invalid trace contexts
     InvalidTC = #span_ctx{trace_id = 0,