fix: handle undefined in cmdt record creation

[mshanemc]

it was possible to get undefined in the customField xml.

This change skips fields where the user provided no value.

@W-13055835@
forcedotcom/cli#2074
forcedotcom/cli#1714

closes Known Issue https://issues.salesforce.com/issue/a028c00000j5kO8AAI/custom-metadata-forcecmdtrecordinsert-command-is-creating-undefined-types-for-null-values-in-csv-file

[shetzel]

QA:
Ran the repro steps from the linked GH issue (created Dashboard_ID, etc.) and then ran:
sf cmdt record insert --filepath list.csv --typename Dashboard_ID --namecolumn Random_Value_Name__c (i.e., without the fix) and the command failed to create a valid Dashboard_ID type.
~/plugin-custom-metadata/bin/run.js cmdt record insert --filepath list.csv --typename Dashboard_ID --namecolumn Random_Value_Name__c (i.e., with the fix) and the command created a valid Dashboard_ID type.