Inaccurate Error Message When Running flutter build ipa Without Internet Connection

[leo271]

When attempting to run flutter build ipa, I encountered an error message stating Null check operator used on a null value during the phase where Dart packages are downloaded. This issue occurred when my machine was not connected to the internet. Here's the relevant log file;
pub_log.txt

Expected Behavior:
The error message should explicitly indicate a connectivity issue, such as Internet connection error.

Actual Behavior:
The error message is misleading as it indicates a null check operation failure, which doesn't clearly reflect the underlying issue of a missing internet connection.

Steps to Reproduce:

1. Disconnect from the internet.
2. Run flutter build ipa.
3. Observe the error message.

After reconnecting to the internet, the problem was resolved, indicating that the issue was indeed related to the network connectivity. This misleading error message could lead to unnecessary debugging efforts focusing on the wrong aspect of the problem.

[sigurdm]

Oh - maybe we should avoid reading advisories when in --offline mode.

cc @szakarias .

[sigurdm]

But basically I think this is a duplicate of #4269