panic: runtime error: invalid memory address or nil pointer dereference (InfluxDB 2.0.4)

__Steps to reproduce:__
Unsure what causes the runtime error.

__Expected behavior:__
Not sure if related, but there's an alert I expected to be triggered but it isn't.

__Actual behavior:__

__Environment info:__

* System info: Host: `Linux 4.14.225-168.357.amzn2.x86_64 x86_64`, Docker image: `Linux 4.14.225-168.357.amzn2.x86_64 x86_64`
* InfluxDB version: `InfluxDB 2.0.4 (git: 4e7a59bb9a) build_date: 2021-02-08T17:47:02Z`
* Other relevant environment details: Docker image from `quay.io/influxdb/influxdb:v2.0.4`

__Config:__

```
docker_container:
name: influxdb
image: quay.io/influxdb/influxdb:v2.0.4
command: --reporting-disabled --http-bind-address=:9999 --engine-path=/data/engine --bolt-path=/data/influxd.bolt
```

__Logs:__

Following text is seen repeating thrice in the logs.

```
ts=2021-04-12T13:51:55.306493Z lvl=info msg="Retention policy deletion check (start)" log_id=0TBMC3CG000 service=retention op_name=retention_delete_check op_event=start
ts=2021-04-12T13:51:55.306670Z lvl=info msg="Retention policy deletion check (end)" log_id=0TBMC3CG000 service=retention op_name=retention_delete_check op_event=end op_elapsed=0.185ms
ts=2021-04-12T13:57:53.563922Z lvl=info msg="Execute source panic" log_id=0TBMC3CG000 service=storage-reads error="panic: runtime error: invalid memory address or nil pointer dereference" stacktrace=

goroutine 156462181 [running]:

runtime/debug.Stack(0xc0011a0240, 0x39feb00, 0x2757856)
/usr/local/go/src/runtime/debug/stack.go:24 +0x9f
github.com/influxdata/flux/execute.(*executionState).do.func1.1(0xc00e4a9a80)
/home/circleci/go/pkg/mod/github.com/influxdata/flux@v0.104.0/execute/executor.go:279 +0x1e5
panic(0x2395400, 0x4bbf8e0)
/usr/local/go/src/runtime/panic.go:969 +0x175
github.com/influxdata/influxdb/v2/v1/services/storage.(*Store).measurementFields.func1(0x0, 0x0)
/home/circleci/go/src/github.com/influxdata/influxdb/v1/services/storage/store.go:546 +0x22
panic(0x23b14a0, 0xc016a5ab40)
/usr/local/go/src/runtime/panic.go:969 +0x175
github.com/influxdata/influxdb/v2/v1/services/storage.(*Store).measurementFields(0xc000a21950, 0x3a4f080, 0xc016a5a810, 0xc010853bb8, 0x0, 0x0, 0x0, 0x0)
/home/circleci/go/src/github.com/influxdata/influxdb/v1/services/storage/store.go:549 +0x2ea
github.com/influxdata/influxdb/v2/v1/services/storage.(*Store).TagValues(0xc000a21950, 0x3a4f080, 0xc016a5a810, 0xc016a5ab10, 0x0, 0xc008ae0cb0, 0x40f6d8, 0x20)
/home/circleci/go/src/github.com/influxdata/influxdb/v1/services/storage/store.go:406 +0x3c5
github.com/influxdata/influxdb/v2/storage/flux.(*tagValuesIterator).Do(0xc00e4a9d00, 0xc008a39bc0, 0x498546, 0x607451e5)
/home/circleci/go/src/github.com/influxdata/influxdb/storage/flux/reader.go:949 +0x162
github.com/influxdata/influxdb/v2/query/stdlib/influxdata/influxdb.(*Source).processTables(0xc00fb90b60, 0x3a4f080, 0xc016a5a810, 0x3a29e00, 0xc00e4a9d00, 0x167520f4f47e8413, 0x167520f4f4129b08, 0x0)
/home/circleci/go/src/github.com/influxdata/influxdb/query/stdlib/influxdata/influxdb/source.go:76 +0x94
github.com/influxdata/influxdb/v2/query/stdlib/influxdata/influxdb.(*readTagValuesSource).run(0xc00fb90b60, 0x3a4f080, 0xc016a5a810, 0x4426b9d2a158e946, 0x2741fa4)
/home/circleci/go/src/github.com/influxdata/influxdb/query/stdlib/influxdata/influxdb/source.go:491 +0x178
github.com/influxdata/influxdb/v2/query/stdlib/influxdata/influxdb.(*Source).Run(0xc00fb90b60, 0x3a4f080, 0xc016a5a810)
/home/circleci/go/src/github.com/influxdata/influxdb/query/stdlib/influxdata/influxdb/source.go:56 +0x33c
github.com/influxdata/flux/execute.(*executionState).do.func1(0x3a4f080, 0xc016a5a810, 0xc00f4d8360, 0xc00e4a9a80, 0x3a50c00, 0xc00fb90b60)
/home/circleci/go/pkg/mod/github.com/influxdata/flux@v0.104.0/execute/executor.go:284 +0x1da
created by github.com/influxdata/flux/execute.(*executionState).do
/home/circleci/go/pkg/mod/github.com/influxdata/flux@v0.104.0/execute/executor.go:254 +0xf8

ts=2021-04-12T14:06:02.146690Z lvl=info msg="index opened with 8 partitions" log_id=0TBMC3CG000 service=storage-engine index=tsi
ts=2021-04-12T14:06:02.147532Z lvl=info msg="Reindexing TSM data" log_id=0TBMC3CG000 service=storage-engine engine=tsm1 db_shard_id=200
ts=2021-04-12T14:06:02.147562Z lvl=info msg="Reindexing WAL data" log_id=0TBMC3CG000 service=storage-engine engine=tsm1 db_shard_id=200
ts=2021-04-12T14:21:55.306502Z lvl=info msg="Retention policy deletion check (start)" log_id=0TBMC3CG000 service=retention op_name=retention_delete_check op_event=start
ts=2021-04-12T14:21:55.306677Z lvl=info msg="Retention policy deletion check (end)" log_id=0TBMC3CG000 service=retention op_name=retention_delete_check op_event=end op_elapsed=0.186ms
```

__Performance:__

[iostat.txt](https://github.com/influxdata/influxdb/files/6305296/iostat.txt)

Start with v1/services/storage/store.go, especially measurementFields and TagValues, then trace the call through storage/flux/reader.go and query/stdlib/influxdata/influxdb/source.go. Reproduce the alert-related query from the InfluxDB 2.0.4 environment if possible and determine the conditions behind the nil-pointer panic. The issue does not define clear expected behavior beyond avoiding the panic and investigating the missing alert.

Written by the indexing model from the issue text.