[NPU] Add checks for potential null pointers (#33547)
### Details:
- check for `zeGraphExt` in `Graph` and `WeightlessGraph`
- check for `vclCompilerPtr` in `PluginCompilerAdapter`
### Tickets:
- *C-179040*
- *C-179041*
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Signed-off-by: alexandruenache1111 <alexandru.enache@intel.com>
