Fix NullPointerException in ResponseDefinition.getProxyUrl()

[mfruizs]

Added safe programming when we instantiate ResponseDefinition class and use getProxyUrl() method when originalRequest field is set to null

References

• Issue reported: Issue-2489

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[oleg-nenashev]

LGTM. We should enforce static analysis in WireMock APIs, because it will be difficult to maintain the extension eco system otherwise