Handle `none` value for OTEL_PROPAGATORS #470

Job	Run time
opentelemetry-api	39s
opentelemetry-exporter-opencensus	41s
opentelemetry-exporter-otlp-combined	47s
opentelemetry-exporter-otlp-proto-common	50s
opentelemetry-exporter-otlp-proto-grpc	48s
opentelemetry-exporter-otlp-proto-http	53s
opentelemetry-exporter-prometheus	36s
opentelemetry-exporter-zipkin-combined	42s
opentelemetry-exporter-zipkin-json	35s
opentelemetry-exporter-zipkin-proto-http	42s
opentelemetry-getting-started	59s
opentelemetry-opencensus-shim	43s
opentelemetry-opentracing-shim	39s
opentelemetry-propagator-b3	36s
opentelemetry-propagator-jaeger	37s
opentelemetry-proto-protobuf5	34s
opentelemetry-sdk	45s
opentelemetry-semantic-conventions	31s
opentelemetry-test-utils	41s
	13m 18s

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