Added basic type checks in java.lsp.server config change listeners

[sid-srini]

• Added checks for null and required JsonElement type in the registered config change listeners in WorkspaceService.
• Included a check for blank input defaultPlatformOverride in AbstractJavaPlatformProviderOverride to avoid expensive initialization in such cases.
• Included a unit test for config values reset to null being received as a JsonNull value.

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