fix(context): guard nil local config before context switch

[Rahulatram321]

Description

This PR fixes a crash path in the microcks context command when no local config exists yet.

Problem

When running microcks context <context-name> on a fresh machine (or after config cleanup), localCfg can be nil.
The command dereferences localCfg (localCfg.CurrentContext) without a nil guard, which can lead to a nil pointer panic instead of a user-friendly message.

Changes

• Added a defensive localCfg == nil check in cmd/context.go before first dereference in the context-switch flow.
• Preserved current behavior for:
  • microcks context (no args): still lists contexts / prints existing output path.
  • microcks context --delete ...: unchanged.

Result

• No panic when config is missing.
• Users now get a clear, deterministic error path instead of a runtime crash.

[github-actions]

👋 @Rahulatram321

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