fix(components/plc4x): fix NPE when Camel can not establish connection

[jmeierhofer]

Description

When Camel can not establish the connection to the S7 PLC, the component is throwing follow-up exceptions which are causing the application start up to fail and which hide the real connection issue.

• Added isConnected() to endpoint which checks if connection is null. This method is public to allow external logic to check if endpoint is really connected.
• Moved canWrite() call to be checked only when connected.
• Added additional canWrite() check after reconnectIfNeeded().

Steps to reproduce:

• Add Camel route with S7 driver
• PLC is not available
• Connection failure
• Additional NullPointerExceptions are thrown

Target

• I checked that the commit is targeting the correct branch (note that Camel 3 uses camel-3.x, whereas Camel 4 uses the main branch)

Tracking

• If this is a large change, bug fix, or code improvement, I checked there is a JIRA issue filed for the change (usually before you start working on it).

Apache Camel coding standards and style

• I checked that each commit in the pull request has a meaningful subject line and body.

• I have run mvn clean install -DskipTests locally from root folder and I have committed all auto-generated changes.

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[jmeierhofer]

@oscerd I think the maven download failed during the JDK17 build. Can you please trigger the 2 builds again?

[oscerd]

/component-tests plc4x

Result ✅ The tests passed successfully

[github-actions]

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