Only set kql custom indicator filter if it exists

[nick-benoit]

Fixes nil pointer error due to optional filter (docs) not being set in indicator properties SLOs_indicator_properties_custom_kql

example:

		                kql_custom_indicator {
		                	index = "my-index-%s"
		                	good = "latency < 300"
		                	total = "*"
		                	timestamp_field = "custom_timestamp"
		                }
				

Note that this only seems to be a problem for kql_custom_indicator because it is the only case where Filter is a struct (*KqlWithFilters) instead of a *string

Previously the provider would fail with the following error:

Error: Plugin did not respond

The plugin encountered an error, and failed to respond to the
plugin6.(*GRPCProvider).ReadResource call. The plugin logs may contain more
details.

Error: Request cancelled

The plugin6.(*GRPCProvider).UpgradeResourceState request was cancelled.

Stack trace from the terraform-provider-elasticstack_v0.11.18 plugin:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0x1222df4]

goroutine 490 [running]:
github.com/elastic/terraform-provider-elasticstack/internal/kibana.resourceSloRead({0x1b1ad10, 0x400083e700}, 0x400088eab0, {0x1781c00?, 0x40004b46c0?})
        github.com/elastic/terraform-provider-elasticstack/internal/kibana/slo.go:1022 +0xad4
github.com/hashicorp/terraform-plugin-sdk/v2/helper/schema.(*Resource).read(0x40003dad00, {0x1b1ac68, 0x4000fd7ce0}, 0x400088eab0, {0x1781c00, 0x40004b46c0})
        github.com/hashicorp/terraform-plugin-sdk/[email protected]/helper/schema/resource.go:866 +0xe0
github.com/hashicorp/terraform-plugin-sdk/v2/helper/schema.(*Resource).RefreshWithoutUpgrade(0x40003dad00, {0x1b1ac68, 0x4000fd7ce0}, 0x4000903ad0, {0x1781c00, 0x40004b46c0})
        github.com/hashicorp/terraform-plugin-sdk/[email protected]/helper/schema/resource.go:1162 +0x3d4
github.com/hashicorp/terraform-plugin-sdk/v2/helper/schema.(*GRPCProviderServer).ReadResource(0x400000cdb0, {0x1b1ac68?, 0x4000fd7bc0?}, 0x4000835180)
        github.com/hashicorp/terraform-plugin-sdk/[email protected]/helper/schema/grpc_provider.go:915 +0x92c
github.com/hashicorp/terraform-plugin-mux/tf5to6server.v5tov6Server.ReadResource({{0x1b370b8?, 0x400000cdb0?}}, {0x1b1ac68, 0x4000fd7bc0}, 0x0?)
        github.com/hashicorp/[email protected]/tf5to6server/tf5to6server.go:188 +0x58
github.com/hashicorp/terraform-plugin-mux/tf6muxserver.(*muxServer).ReadResource(0x400028cf30, {0x1b1ac68?, 0x4000fd78f0?}, 0x4000835130)
        github.com/hashicorp/[email protected]/tf6muxserver/mux_server_ReadResource.go:35 +0x178
github.com/hashicorp/terraform-plugin-go/tfprotov6/tf6server.(*server).ReadResource(0x4000185180, {0x1b1ac68?, 0x4000fd70e0?}, 0x40002a8700)
        github.com/hashicorp/[email protected]/tfprotov6/tf6server/server.go:863 +0x1f8
github.com/hashicorp/terraform-plugin-go/tfprotov6/internal/tfplugin6._Provider_ReadResource_Handler({0x17d7dc0, 0x4000185180}, {0x1b1ac68, 0x4000fd70e0}, 0x4000e23680, 0x0)
        github.com/hashicorp/[email protected]/tfprotov6/internal/tfplugin6/tfplugin6_grpc.pb.go:753 +0x1b8
google.golang.org/grpc.(*Server).processUnaryRPC(0x40002a4000, {0x1b1ac68, 0x4000fd7050}, 0x4000ebe780, 0x400023dcb0, 0x2b9c2f8, 0x0)
        google.golang.org/[email protected]/server.go:1431 +0xbd0
google.golang.org/grpc.(*Server).handleStream(0x40002a4000, {0x1b1bdf8, 0x400019e820}, 0x4000ebe780)
        google.golang.org/[email protected]/server.go:1842 +0x858
google.golang.org/grpc.(*Server).serveStreams.func2.1()
        google.golang.org/[email protected]/server.go:1061 +0x74
created by google.golang.org/grpc.(*Server).serveStreams.func2 in goroutine 44
        google.golang.org/[email protected]/server.go:1072 +0x120

Error: The terraform-provider-elasticstack_v0.11.18 plugin crashed!

This is always indicative of a bug within the plugin. It would be immensely
helpful if you could report the crash with the plugin's maintainers so that it
can be fixed. The output above should help diagnose the issue.

Rel: #1098

[tobio]

LGTM, can you add a changelog entry for this too?