fix: nil pointer dereference in conversion webhook

[drivebyer]

Description
The operator log show:

2024/02/19 03:12:45 http: panic serving 10.6.231.10:55106: runtime error: invalid memory address or nil pointer dereference
goroutine 2997 [running]:
net/http.(*conn).serve.func1()
    /usr/local/go/src/net/http/server.go:1868 +0xb9
panic({0x194d4a0?, 0x2be6c70?})
    /usr/local/go/src/runtime/panic.go:920 +0x270
github.com/OT-CONTAINER-KIT/redis-operator/api/v1beta1.(*RedisSentinel).ConvertFrom(0xc000345a00, {0x1e4f3e8?, 0xc0006f6b40})
    /workspace/api/v1beta1/redissentinel_conversion.go:125 +0x4cb
sigs.k8s.io/controller-runtime/pkg/webhook/conversion.(*webhook).convertObject(0xc00004ef20?, {0x1e48f10, 0xc0006f6b40}, {0x1e49258, 0xc000345a00})
    /go/pkg/mod/sigs.k8s.io/[email protected]/pkg/webhook/conversion/conversion.go:140 +0x376
sigs.k8s.io/controller-runtime/pkg/webhook/conversion.(*webhook).handleConvertRequest(0xc00004ef20, 0xc000963000)
    /go/pkg/mod/sigs.k8s.io/[email protected]/pkg/webhook/conversion/conversion.go:105 +0x1c5
sigs.k8s.io/controller-runtime/pkg/webhook/conversion.(*webhook).ServeHTTP(0x1e44b00?, {0x7f603ce4ad30?, 0xc0001c0730}, 0xc0008fea00)
    /go/pkg/mod/sigs.k8s.io/[email protected]/pkg/webhook/conversion/conversion.go:72 +0xf6
sigs.k8s.io/controller-runtime/pkg/webhook/internal/metrics.InstrumentedHook.InstrumentHandlerInFlight.func1({0x7f603ce4ad30, 0xc0001c0730}, 0x1e4f900?)
    /go/pkg/mod/github.com/prometheus/[email protected]/prometheus/promhttp/instrument_server.go:60 +0xcb
net/http.HandlerFunc.ServeHTTP(0x1e4f9b8?, {0x7f603ce4ad30?, 0xc0001c0730?}, 0xc0005418a0?)
    /usr/local/go/src/net/http/server.go:2136 +0x29
github.com/prometheus/client_golang/prometheus/promhttp.InstrumentHandlerCounter.func1({0x1e4f9b8?, 0xc0001fa620?}, 0xc0008fea00)
    /go/pkg/mod/github.com/prometheus/[email protected]/prometheus/promhttp/instrument_server.go:147 +0xb6
net/http.HandlerFunc.ServeHTTP(0x701406?, {0x1e4f9b8?, 0xc0001fa620?}, 0x410725?)
    /usr/local/go/src/net/http/server.go:2136 +0x29
github.com/prometheus/client_golang/prometheus/promhttp.InstrumentHandlerDuration.func2({0x1e4f9b8, 0xc0001fa620}, 0xc0008fea00)
    /go/pkg/mod/github.com/prometheus/[email protected]/prometheus/promhttp/instrument_server.go:109 +0xc2
net/http.HandlerFunc.ServeHTTP(0x10?, {0x1e4f9b8?, 0xc0001fa620?}, 0xc000962fac?)
    /usr/local/go/src/net/http/server.go:2136 +0x29
net/http.(*ServeMux).ServeHTTP(0x410725?, {0x1e4f9b8, 0xc0001fa620}, 0xc0008fea00)
    /usr/local/go/src/net/http/server.go:2514 +0x142
net/http.serverHandler.ServeHTTP({0x1e497d0?}, {0x1e4f9b8?, 0xc0001fa620?}, 0x6?)
    /usr/local/go/src/net/http/server.go:2938 +0x8e
net/http.(*conn).serve(0xc000377c20, {0x1e5cb48, 0xc00017c240})
    /usr/local/go/src/net/http/server.go:2009 +0x5f4
created by net/http.(*Server).Serve in goroutine 66
    /usr/local/go/src/net/http/server.go:3086 +0x5cb

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist

• Tests have been added/modified and all tests pass.
• Functionality/bugs have been confirmed to be unchanged or fixed.
• I have performed a self-review of my own code.
• Documentation has been updated or added where necessary.

Additional Context

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 32.78%. Comparing base (df2df6d) to head (697c4f9).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #787      +/-   ##
==========================================
+ Coverage   32.76%   32.78%   +0.02%     
==========================================
  Files          19       19              
  Lines        3235     3233       -2     
==========================================
  Hits         1060     1060              
+ Misses       2116     2114       -2     
  Partials       59       59              

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