NonNull Types for Map values can contain null values

[Legion2]

Describe the bug
When deserializing an object into a map of a NonNull value Type, e.g. Map<String, String> there should be no null values in the map. But no exception is thrown when deserializing an object which contains null values into such a map, which then contains null values and cause NPEs.

To Reproduce
Following test demonstrates the bug:

    @Test
    fun test() {
        val objectMapper = ObjectMapper()
        objectMapper.registerKotlinModule()
        assertThrows<RuntimeException> {
            objectMapper.readValue<Map<String, String>>("""
            {
                "key1": "value",
                "key2": null
            }
            """.trimIndent())
        }
    }

Expected behavior
When the value type of a map declares to be non nullable there should be no null values at runtime, otherwise it breaks kotlin's Null Safety. Instead an error should be reported when there is a null value.

Versions
Kotlin: 1.3.72
Jackson-module-kotlin: 2.11.3
Jackson-databind: 2.11.3

[k163377]

Basically, this can only be prevented by enabling the StrictNullChecks option (note that enabling this option currently has a significant performance penalty).
Also, for readValue<Map<String, String>>, no null checks are performed even if the StrictNullChecks option is enabled, because of the problem in and #399.

This issue is closed as a duplicate of #399.