[27.x backport] plugins: don't panic on Close if PluginServer nil

[vvoland]

• backport: plugins: don't panic on Close if PluginServer nil #5327
• fixes[BUG] SIGSEGV: segmentation violation code=0x2 addr=0x0 pc=0x1007d0440 #5317

- What I did

We had a report in fixes#5317 of a panic at

cli/cli-plugins/socket/socket.go

Line 111 in 87c6624

pl.mu.Lock()

Looks like this will happen if the call to create a PluginServer fails and returns nil, since we're not checking for nilness before attempting to call Close:

cli/cmd/docker/docker.go

Lines 247 to 251 in 87c6624

	deferfunc() {
	// Close the server when plugin execution is over, so that in case
	// it's still open, any sockets on the filesystem are cleaned up.
	_=srv.Close()
	}()

cli/cmd/docker/docker.go

Line 278 in 87c6624

_=srv.Close()

- How I did it

Added a nil check to PluginServer.Close(). We could also have addressed this at the caller sites, but this seems more robust for little cost.

- How to verify it

- Description for the changelog

Fixed issue where main CLI process would sometimes panic while running a plugin.

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 61.05%. Comparing base (40707e1) to head (984ef90).
Report is 1 commits behind head on 27.x.

Additional details and impacted files

@@ Coverage Diff @@## 27.x #5337 +/- ##
==========================================
- Coverage 61.49% 61.05% -0.44% 
==========================================
Files 299 296 -3 Lines 20867 20861 -6 ==========================================
- Hits 12832 12737 -95 - Misses 7120 7207 +87 - Partials 915 917 +2 

[thaJeztah]

LGTM