Stream Analytics Job LastOutputEventTime defaults to JobStartTime if no output event has been processed before

**Context**
Currently, Stream Analytics Job `OutputStartMode` can only be set to `LastOutputEventTime` if the event has been processed before. Otherwise, the following error occurs:

`"LastOutputEventTime must be available when OutputStartMode is set to LastOutputEventTime. Please make sure at least one output event has been processed. "`

This means that the user needs to initially set `OutputStartMode` to `JobStartTime` and later to `LastOutputEventTime` if that's the desired behaviour. As a consequence, the config file responsible for deploying ASA job needs to be tweaked after the first deployment which is undesirable in the prod environment.

**Proposed behaviour**
The proposed behaviour is to default to `JobStartTime` when the user has chosen `LastOutputEventTime` setting and no event has been processed before.

**Broken documentation formatting**
Also, [the documentation](https://docs.microsoft.com/en-us/powershell/module/azurerm.streamanalytics/start-azurermstreamanalyticsjob?view=azurermps-6.13.0) is misleading and not properly formatted which may suggest the proposed behaviour:


Start with the Start-AzureRmStreamAnalyticsJob entry point and its OutputStartMode handling. Reproduce the no-output-event case, then verify that LastOutputEventTime uses JobStartTime and that the linked documentation is corrected to describe and format this behavior.

Written by the indexing model from the issue text.