fix(modem): Add asserts and params error checks on create path

[david-cermak]

Follow up from #1102

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Note

Low Risk
Small guardrails on creation only; behavior for valid callers is unchanged and invalid null args now fail safely instead of undefined behavior.

Overview
Adds defensive null-pointer validation on modem initialization so invalid callers fail early instead of dereferencing null configs or DTE handles.

On the C++ factory path, create_vfs_dte, create_uart_dte, and shared create_modem_dce now use ESP_MODEM_THROW_IF_FALSE for null dte/dce config and null dte before building terminals or DCE instances (still surfaced as nullptr via TRY_CATCH_RET_NULL when exceptions are enabled).

On the C API, esp_modem_new_dev returns nullptr immediately if dte_config or dce_config is null, before allocating the wrapper or calling into UART DTE creation.

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