EnvelopedData.decrypt() breaks if UKM is absent when using ECDH

[gnarea]

This bug doesn't affect me but I found it by chance whilst debugging #334: If you try to decrypt an EnvelopedData that doesn't have a user keying material (UKM) and you're using ECDH, you'd get the following error:

TypeError: Cannot read property 'toBER' of undefined

    at LocalConstructedValueBlock.toBER (/home/gus/repos/relaynet-core-js/node_modules/asn1js/src/asn1.js:1247:35)
    at Constructed.toBER (/home/gus/repos/relaynet-core-js/node_modules/asn1js/src/asn1.js:931:45)
    at LocalConstructedValueBlock.toBER (/home/gus/repos/relaynet-core-js/node_modules/asn1js/src/asn1.js:1247:35)
    at Sequence.toBER (/home/gus/repos/relaynet-core-js/node_modules/asn1js/src/asn1.js:931:45)
    at /home/gus/repos/relaynet-core-js/node_modules/pkijs/src/EnvelopedData.js:1429:45
    at processTicksAndRejections (internal/process/task_queues.js:97:5)
    at Object.<anonymous> (/home/gus/repos/relaynet-core-js/src/integration_tests/pkijs_bug.test.ts:59:21)

That's because ukm is assumed to be present:

PKI.js/src/EnvelopedData.js

Line 953 in 699ac6e

entityUInfo: recipientInfo.value.ukm,

PKI.js/src/ECCCMSSharedInfo.js

Line 187 in 682ef8d

if("entityUInfo" in this)

But it doesn't have to be according to RFC 5753:

ukm MAY be present or absent. However, message originators SHOULD include the ukm.

Note that openssl cms -encrypt doesn't add a UKM, and BouncyCastle would only do it if explicitly set.